– A sidenote on Pizza Hawaii –
In our daily lives, we are confronted now and then if we should do things differently. Even when we speak about benign things like the maintenance of the car and a friend suggests a different way of cleaning it. When we bring food to a party and discuss different ingredients for the same dish. Aside from the discussion about whether or not pineapple belongs on a pizza, how do you objectively determine if it is good to progress, adopt new methods and ideas? In terms of culinary progress, it is evidently easy. Either you like it or not and hence, you progress or not. But when you take decisions on a different level it becomes less simple.
How do you determine if the benefit is worth the effort of restructuring a traffic system of a city or the educational program for a whole country? Discussions on this scale tend to consume a lot of time and energy from people involved and are inherently slow. But how come that big companies determine if it is worthwhile opening up a new store in a city district on a day to day basis? How do traders at the stock market make decisions about the outcome of investment in a matter of seconds and minutes?
Even hundreds of years ago, farmers had to cope with the questions of investment. When considering buying additional livestock, you are confronted with spending more resources and advancing your stables with the prospect of gaining a higher income. Although there might be enough income already, people tend to strive for growth. Hence, these farmers are outweighing the financial effort against the prospective income. As basic as this sounds even today in modern prediction models the same old principle is at work. This process of outweighing effort against benefit can be perceived as a very simple question. What is the minimum effort, that I can undertake, to achieve my goal? The answer to this question then gives us a value or a guideline when or how we should progress.
So what we have is a like a linear system with a number of variables that, if solved, yields the value we require to make our decision. And exactly this was the approach by Georg Danzig in 1947. He developed the so-called “Simplex-Algorithm” that is capable of solving such questions with a limited amount of iteration steps. Thereby a complex problem can be disassembled into several variables with different impact factor and processed by this method.
These simplex-algorithms as a subclass of linear optimization processes are essential for the prediction of economic development of whole countries as well as freight transport and management on a global scale. With modern technology, these models will be able to suggest the quickest, cheapest or even the most environmentally friendly way of transporting goods, depending on what you prioritize. Since the establishment of such prediction methods a lot of research, development and refinement produced a wide variety of models using linear optimization, heuristics or even randomization.
This is not surprising considering that some questions are just more complex than others. Take for example the paper making industry, where a product can have so many different specifications: material, thickness, size, binding, coloration, surface processing, water sign, and many more options. On top, the manufacturing process is also quite individual depending on the specification of the desired product. There is simply an incredible amount of combinations and therefore variables which to account for.
Therefore, such complex problems have to be divided in to groups of problems and sub problems which takes more time and resources to be solved. Besides, straying from the linear dimension of the non-linear optimization enables solving highly complex systems. This however, can reach a level where people can not even trace back all logical decisions as is already the case for the use of artificial intelligence in stock market decisions. These AIs take a known working approach and refine it to the maximum even for complex systems. On the other hand, a lot of man-made systems also work by this principle. The scientific community in itself is thriving from optimisation and advancement aside from answering fundamental questions.
One field that resists vigorously against all approaches for mere rational optimization however is food. What was formerly known as space food (a dry powder that contains all necessary nutrients) is now commercially available in a wide variety.
But all of these mixtures, that claim to contain all the nutrients in the perfect balance that we need, have one problem in common. The balance of nutrients is based on scientific findings which are just an average and do not account for everyone and they are not definite. There may be nutrients we need that we have not identified yet. Without the addition of aromas they often do not taste even if our taste buds might register the presence of all our required nutrients. On the other hand, the taste is highly dependent on nutrients. This makes food a highly complex system with a lot of variables to atone for.
There is a place that just is not always so rational when it comes to decision making and that is our mind. Therefore, it is not surprising that Pizza Hawaii, which can even be nutritionally favorable over some other pizzas, is not as popular. Even when considering more variables as pricing, it is a famous example of the irrationality of human decisions, since we simply have different taste and do not always make decisions on a mere objective basis. We base decisions on values we establish for ourselves and a tradition that ensures a good taste might just be more important than experimenting with ingredients on a pizza. This is exactly the weak point of such prediction models since an algorithm might be perfectly capable of suggesting to us the perfect company, living place, and even partner but still fails to really grasp what we expect from life on a personal basis.
In principle, we can say it is always good to embrace progress, but in some regards, it is quite acceptable to stick to your old guns. We can use logical tools like the simplex-algorithm to help us determine the course of very complex systems like governing traffic. But these tools can not ultimately solve the question of what we should eat or how we want to live together as a society. Let alone, what equality means and how we ensure it. These are complex questions that we have to answer the old way by time-consuming but worthwhile debates.
— Kevin Machel
 “www.wikipedia.org/wiki/Simplex-Verfahren”, 2020.
 Smith, S., The simplex method and evolutionary algorithms.International Conference on Evolutionary Computation Proceedings. , 1998, pp. 799-804.
 Lee, S., Kang, Y. and Prabhu, V. V., Smart logistics: distributed control of green crowdsourced parcel services, International Journal of Production Research, 54(23), 2016, 6956-6968.
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 Popkova, Elena G.and Parakhina, Valentina N., “The Future of the Global Financial System: Downfall or Harmony”, Springer International Publishing ,2019, pp. 939-946,
 “www.wired.co.uk/article/huel-soylent-meal-replacement-drinks”, 2020.
 Beeler, J. A., Mccutcheon, J. E., Cao, Z. F. H., Murakami, M., Alexander, E., Roitman, M. F., and Zhuang, X., Taste uncoupled from nutrition fails to sustain the reinforcing properties of food, European Journal of Neuroscience, 2012, 36(4), pp. 2533-2546.
 “www.pccmarkets.com/product-info/deli/pcc-deli-cheese-pizza-620899”, 2020.
 “www.pccmarkets.com/product-info/deli/pcc-deli-hawaiian-pizza-697590”, 2020.
 Liu, Y., Zhang, L., Nie, L., Yan, Y. and Rosenblum, D. S., “Fortune teller: Predicting your career path”, 30th Conference on Artificial Intelligence, 2016, pp. 201-207.
 A. M. Lukatskii and G. V. Fedorova, “Algorithms and software for studying the impact of fuel and energy prices on the economy of the Russian federation”, 10th Int. Conf. Management of Large-Scale System Development (MLSD), 2017, pp. 1-5, doi: 10.1109/MLSD.2017.8109653.
Right? If it runs like a charm, no one wants to improve a process for the worst. But sometimes old habits stick at the expense of progress. We collect opinions, experiences, and failures or successes in this specific topic. Please use the text box and send us your contribution (ca 250 words) until Sep. 13th 2020.
The contributions will be collected and published after reviewing in our next issue.
The editorial board of the Journal of Unsolved Questions
2 You might have encountered the phenomenon: You have a faucet running, for instance, while filling your bathtub, and quite a bit of water is flowing out. Then you want to stop the flow and quickly turn the valve to block the water. Suddenly, in the very moment you close the valve, you hear a banging sound in the walls around you. Sometimes it is rather quiet, sometimes it can be scarily loud. But what is actually causing the sounds and is it something you should worry about?
It turns out that the answer to this question is one of the most relevant design considerations for civil engineering.1,2 It all roots in a fundamental property of liquids: not being compressible. You may have noticed that a balloon filled with air can be compressed quite a bit while one filled with water only changes shape and cannot be made smaller. This very principle is the core of hydraulics, i.e. using the volume of a liquid like water or oil to transfer force in a system.
Maybe you are wondering now how this affects your pipes, considering that the volume of the pipe always stays the same. Nobody (hopefully) compresses the pipes in your walls. There is, however, also another important aspect to a liquid: its momentum. Once a liquid starts to move through a pipe it builds momentum just like a car or a train build momentum when they move (see Fig. 1). As long as the flow can continue undisturbed no ill effects occur and similarly, nothing happens if the flow is slowly brought to a halt by gradually closing the valve. To stay in our analogy, this would correspond to a train of cars slowing down when a road gradually narrows from two lanes to one and eventually to a road blockage.
The interesting effects happen when we force the flow of liquid to stop suddenly with a valve, essentially causing an accident on our road that causes all the cars to slam into the blockage. In this case, a rather large amount of inertia must be dissipated into the valve and the walls of the pipe causing a large pressure spike. If the pipe can move like a free garden hose (e.g. old pipes that are not properly fixed) this can lead to a sudden jerk and might loosen connections. The story is not so easy, though, for pipes that are fixed in your walls or buried beneath streets: those usually cannot move. As a result, the full force of the pressure acts on the valve and the walls of the pipe – causing the banging sound in your bathroom. The effect is officially called ‘water hammer’.3
But before you start wondering now if your pipes will one day burst and start flooding your apartment: civil engineering has developed a series of safety measured to prevent this from happening in our daily life. To understand how, it is important to know that the pressure spike caused by the sudden restriction in water flow is directly related to the flow rate. In our car analogy this would correspond to the relation between the number of lanes on the road and the number of cars that have to pass through a road segment at any given time. Together this dictates the speed the cars are traveling with and their cumulative inertia. In simple words: fast-moving cars on a narrow street do have a much harder time to slow down than a larger number of slower cars. Therefore, many supply pipes have large diameters, causing the water to flow slowly and thus avoiding the pressure spikes if the flow is restricted by one of the recipients. Additionally, air or spring-loaded pressure relief valves can engage to dissipate a dangerous spike without causing damage and water suppliers usually have build-in safety measures to make sure pumps are not starting up too quickly.
All in all, you usually do not have to worry about a slight banging sound caused by quickly closing a faucet, even though you should avoid it if possible. However, if it is loud or suddenly starts to appear you should be careful to reduce the wear and tear of your pipes or directly ask a pumper. A neat little starting point is Ref. . Anyway, next time you use your sink or shower, think maybe of all the civil engineering that is necessary to let you shower or take a bath anytime at will.
— Kai Litzius
Disclaimer: This article is meant to give an introduction into the physics behind the water hammer. In doubt always ask a specialist if your piping needs to be repaired.
|||A similar effect occurs with steam, the so-called ‘steam hammer’, which is an exceptionally dangerous phenomenon that can lead to steam pipes exploding once too much steam condenses and the condensate gets accelerated through the pipe. See also here for a great explanation: https://www.youtube.com/watch?v=JyvoN1hIqRo|
Absolute nothingness ( Śūnyata ) is one of the most exciting notions in Buddhism. Essentially, it cannot be interpreted anyhow but can be thought of as Ultimate Reality. In Mediterranean tradition, ancient cosmologists introduced another term that sounds more familiar – The Chaos. It was associated with the infinite ocean and expressed an initial state of cosmos in potentia. Not to get numb by the immensity of this semantic unit, we can consider chaos as noise having an infinite spectrum of all conceivable frequencies. And through interaction with external conditions, certain modes manage to become more pronounced as, for example, in the process of stimulated emission build-up in the laser or during the process of natural selection in the theory of evolution.
In the context of road traffic development, we can define the situation in ancient times as the initial chaotic state. As there were no roads as such, the traffic was chaotic. With the evolution of horse-drawn transport, the road map was developing. However, the roads were still only directions along which one could get from one place to another.
The situation changed when engine cars jolted the slow and stagnant horse traffic. Between the man and the road there was no middle link anymore that could choose a better way within the given direction on its own. Nonetheless, engine-drawn transport had an obvious advantage of higher achievable speed. In turn, the desire to move faster and faster required less scattering at the surface roughness, which inevitably resulted in roads getting smoother, i.e., less chaotic. In the meantime, the assembly line was progressing drastically and both factors lead to a dense cloud of potentially fast cars. But people were still scratching their heads why the average speed of the road traffic was not increasing. After a while, they figured out who is to blame in the residual scattering – the interaction of the drivers themselves with each other. With the absence of any predefined rules, everyone had to slow down and likely change the direction to avoid physical interaction with another participant of the traffic. Thus, the necessity of the traffic regulations was obvious.
The first “Convention with respect to the international circulation of motor vehicles” was signed in Paris in 1909. Among others, it contained the sign depicted in Fig. 1, which indicated the road intersection. And naturally, originating from the ship traffic, the habitual priority-to-the-right rule was established to regulate the right-of-way for two vehicles with intersecting directions. Later a set of traffic regulations was complemented with priority signs and traffic lights.
In 1930 Kurt Gödel presented two theorems reflecting insuperable limitations of formal arithmetics. These theorems had a direct relation to the second problem from Hilbert’s list asking for the proof that arithmetics is consistent. The first Gödel’s theorem (in Rosser form) states that within any consistent formal system S, one can come up with expression A that can be neither proved nor disproved. In other words, the axiomatic system S is incomplete. Hao Wang published in his Logical Journey the full text that Gödel had written about his discovery of the incompleteness theorems:
“In the summer of 1930 I began to study the consistency problem of classical analysis. It is mysterious why Hilbert wanted to prove directly the consistency of analysis by finitary methods. I saw two distinguishable problems: to prove the consistency of number theory by finitary number theory and to prove the consistency of analysis by number theory <…> Since the domain of finitary number theory was not well-defined, I began by tackling the second half <…> I represented real numbers by predicates in number theory <…> and found that I had to use the concept of truth (for number theory) to verify the axioms of analysis. By an enumeration of symbols, sentences and proofs within the given system, I quickly discovered that the concept of arithmetic truth cannot be defined in arithmetic. If it were possible to define truth in the system itself, we would have something like the liar paradox, showing the system to be inconsistent <…> Note that this argument can be formalized to show the existence of undecidable propositions without giving any individual instances. (If there were no undecidable propositions, all (and only) true propositions would be provable within the syosmos in potestem. But then we would have a contradiction.) <…> In contrast to truth, provability in a given formal system is an explicit combinatorial property of certain sentences of the system, which is formally specifiable by suitable elementary means…”
Traffic regulations in the context of the 1st Gödel’s theorem
We can consider any set of interrelated rules, including traffic regulations, as a formal axiomatic system where each axiom is not subject to prove and serves as a basis for further deriving the formulas and theorems (or behavior in a traffic situation). Clearly, the traffic regulations are consistent because otherwise, the number of car crashes would be much higher. Hence, according to the 1st Gödel’s theorem, the system is incomplete. This means that there would always exist a situation, which cannot be resolved regardless of the number of regulations (axioms) contained in the system.
The example of such a situation can be observed on the road intersection regulated by priority-to-the-right rule depicted in Fig. 2. Here four vehicles coming from every direction want to pass this intersection each going straight. There is no way to resolve this situation (to derive the formula) within the traffic regulations system and the drivers in every certain situation are supposed to make the decision: who has the priority.
We can incrementally enhance our axiomatic system by introducing another rule to resolve such a dead-end situation. A rule that gives priority to go first, say, to a red car. Again, four red cars on the same road crossing end up with the same confusion. As long as we add the rules (axioms) into the system enumerably, which is the case for the traffic regulations, such situations will always appear. Introducing the priority signs, constant or variable in time, like traffic lights, or topological road junctions (see Fig. 3) can only decrease the probability of this situation emerging.
Nowadays, most of the intersections are controlled (or topologically resolved). And let’s assume that the preposterous situation with four red cars trying to figure out the right-of-way on the uncontrolled intersection hasn’t happened up to the moment in our complex but finite system of road traffic. Hence, the drivers’ behavior seems to be fully governed with the traffic regulations. However, there still is a possibility of an unresolvable situation, namely, if one comes up with an expression: “I’m not going to obey the rules. For the axiomatic system of traffic regulations, this expression serves as a “liar paradox” and cannot be resolved. Thus people had to come up with the penalty system for acceptable performance of the traffic regulations. But again, it is impossible to nullify the probability of such a situation emerging.
Instead of conclusion
The aim of this text was not to establish a solid theory in either mathematics or law, and the presented examples may not be in strict compliance with the described statements. However, the author finds entertaining the fact that there are bridges between different islands of knowledge accumulated by mankind over the infinite ocean of the unknown.
— Sergei Sobolev
 D. Mathers, M. Miller, O. Ando. Self and No-Self: Continuing the Dialogue Between Buddhism and Psychotherapy. 2013 Routledge
 W. Koechner. Solid-State Laser Engineering, 2006 Springer
 C. Darwin. The origin of species by means of natural selection; or, the preservation of favoured races in the struggle for life. 1859 London
 Convention with Respect to the International Circulation of Motor Vehicles. The American Journal of International Law Vol. 4, No. 4, Supplement: Official Documents (Oct., 1910), pp. 316-328
 D. Hilbert. “Mathematical Problems”. Bulletin of the American Mathematical Society. 8 (10): 437–479, 1902.
 Introduction to metamathematics. S. Kleene, 1952 D. Van Nostrand Company, Inc.
 H. Wang. A Logical Journey. From Gödel to Philosophy. 1996 The MIT Press.
Just the thought of getting in touch with or even ingesting urine repels many people. But medical treatment with urine – also called urotherapy – has been a valuable approach in the traditional medicine of many cultures over the last centuries. Usually, endogenous urine is used but animals are also popular sources. The utilization of urine in conventional medicine is not uncommon too. Urokinase, for example, can be isolated from (human) urine and is an important thrombolytic agent. The drug Premarin®, which is used for hormone treatment, contains estrogens that are extracted from the urine of pregnant mares.
Besides milk, camel (i. e. camelus dromedarius) urine plays a special role for desert dwelling people like the Bedouin. Its use was advised by Prophet Mohammed, thus it has found its way into the Islamic prophetic medicine. Apparently, this body liquid cures diseases like tuberculosis, hepatitis, digestion problems, impotence, hemorrhoids, and flatulence, just to name a few. In 2013, one liter of urine from a virgin camel was worth about 15 € (ca 20 USD) in Yemen, where it is not only used for universal medical treatment but also as a cosmetic product for skin and hair care.
Conventional medicine offers plenty of pharmaceutical cancer treatments which are a blessing and a curse for the patients at the same time. Besides the tedious and exhaustive treatment, patients are confronted with severe side-effects including nausea, fatigue, hair loss, inflammation, and temporary immunodeficiency. The demand for alternatives that are at the same time highly effective, easy to use, mild, and in the best case based on renewable resources is therefore very high.
Camel urine has long been claimed to be an efficient cancer treatment but detailed research on its actual potency and effect on human health is scarce. The soothing effect of pure camel urine on digestive problems can sufficiently be explained through its relatively high content of electrolytes like sodium and zinc as found by Al-Attas, in 2009 – a result that certainly might be achieved just as well by drinking a bouillon. Kohrshid et al. were the first to show an inhibiting effect of lyophilized camel urine on carcinoma cells in animals. In 2011, Alhaider et al. found that treatment of murine hepatoma cells (Hepa 1c1c7, i. e. liver cells) with camel urine inhibited the induction of Cyp 1a1 (a well-known cancer-activator) gene expression by TCDD, a potent Cyp 1a1 inducer and a known carcinogen. Among virgin, pregnant, and lactating camels, the virgin’s urine was found to be most potent while the urine of pregnant camels showed the least potency. One year later, Khorsihd et al. showed that the potency of camel urine to reduce a specific type of lung cancer cells (A549) is somewhat dependent on the breed (Majaheem urine was found to be more effective than Magateer urine) and the age of the camels. The depletion of the cancer cells ranged between 85‒93% of the starting cell number.[9,10] The bioactive subfraction PMF which is believed to be responsible for these effects is obtained from lyophilized camel urine (in literature frequently called PM701). Clinical trials on the oral uptake of PM701 fractions showed no negative effects on human health so far. Apparently, the urine contains a high amount of antibodies of such a small size, that they can be easily absorbed by the patient’s digestive system. Other experiments also show antimicrobial effects of camel urine on bacteria and fungi. Aiming at the environmentally friendly substitution of synthetic agents which are usually obtained from complex multistep reactions this approach is most honorable. It is exciting to see that a waste product has the potential to cure severe diseases although much more research must be done on this subject to clearly verify the efficacy. After all, urine is an excretion that contains various less beneficial digestive metabolites, and even toxins that the body wants to get rid of and indisputable evidence for the efficacy and safety of the PM701 fractions are vital.
For those people who are curious enough to try camel urine for whatever reason but are too disgusted by the idea to drink it pure, a solution might be on the way: there are capsules of PM701, or PMF respectively, but they are not yet available on the market. Another alternative might be camel milk which sounds much more enjoyable and is supposed to be a medicine just as magical as camel urine. It is said to “reduce blood sugar […] solve the problems of autism in children, enhance the immunity of the body…” and many more. Alas, some bad news comes from the World Health Organization (WHO) concerning the use of camel milk and urine: shortly after the Middle East respiratory syndrome coronavirus (MERS-CoV) outbreak in Saudi Arabia in the year 2012 dromedary camels were found to be zoonotic transmitters, meaning that the virus is rapidly transferred from animals to humans – just as we experience right now with the latest outbreak of a coronavirus (COVID-19). As a consequence the WHO advises to avoid contact with camels or consuming raw camel milk and urine. This surely dampens the enthusiasm to utilize camel urine and we might have to wait a few years more for some groundbreaking results in cancer research.
‒ Tatjana Dänzer
 “Abstracts of Papers Read”. American Journal of Physiology. Legacy Content., 1952, 171, 704–781.
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 Hammam, A. R. A., Emirates Journal of Food and Agriculture, 2019, 31, 148‒152.
Due to technical improvements during the last years, machines outcompete humans in a couple of specialized tasks: Whereas it can take a human person very long to calculate the square root of a (non-square) number, a computer can finish this calculation at high precision within a fraction of a second. However, there are some areas in which machines still cannot compete with nature (yet). One of them is olfaction: Currently, no device is available that could replace police dogs with the ability to detect trace amounts of molecules. Similarly, farmers sometimes even train pigs to search for truffles hidden in the soil. Of course, the ability to detect relevant molecules in low amounts offers an enormous advantage and is thus subject to extensive optimization by evolution.
How exactly olfaction works in higher organisms has not been known for a long time. Nonetheless, it had been intuitively clear that there must be specific receptors interacting with the corresponding odours. This simple assumption has a remarkable consequence: Since mammals can distinguish a high number of odours, there also must be a high number of different receptors encoded in the genome. Indeed, the two scientists Linda Buck and Richard Axel discovered a comparatively large family of genes encoding for odorant receptors . For this discovery, they were awarded the Nobel Prize in Physiology or Medicine in 2004. The activation of these receptors on the cell surface always results in similar intracellular reactions. If a cell had receptors for different odour molecules on its surface, it could therefore not distinguish these odours. In accordance to this consideration, it turned out that each olfactory cell only carries one type of all the different odorant receptors encoded in its genome. Why exactly this is the case is still not known in detail to date. Even more surprisingly, it even turned out that the axons of cells, which carry the same type of odorant receptor on their surface, end on the same set of cells.
An odour can of course consist of several kinds of molecules. The activation of different combinations of olfactory sensory neurons further increases the number of differentiable odours. A phenomenon seemingly similar to the exclusive expression of a single odorant receptor by an olfactory sensory neuron is the generation of only one type of antigen receptor by immune cells. They achieve this by a complicated recombination of genes, which is clearly not observed in olfactory neurons.
Investigating how a biological structure develops is often very helpful: In a later work, Linda Buck could show that in contrast to mature olfactory neurons, there are multiple mRNAs for different odorant receptors in immature neurons . Why cells of our body can have entirely different morphologies and properties even though they all carry a copy of the same genome is a fundamental question which keeps many biologists busy. It is the differential expression of the genes in a cell, which causes these differences. This gives muscle cells the ability to contract and enables neurons to generate action potentials.
However, all olfactory neurons express a very similar pattern of genes except for their odorant receptor. One of the reasons for the transcription of different amounts of RNAs from different genes is the spatial arrangement of the DNA in the nucleus. Had it not been tightly packed into the nucleus, the DNA in each cell would have a total length of 1.8 m and highly condensed sections of DNA are usually not accessible for transcription into RNA. Stavros Lomvardas, a former member of the group of Richard Axel, could show that DNA segments encoding for odorant receptors on different chromosomes get pulled close to each other in a small spatial region in the nucleus. Interactions between the different DNA segments encoding for odorant receptors could contribute to the exclusive transcription of one specific odorant receptor gene [3,4].
The relevance of the spatial arrangement of the DNA within the nucleus for gene expression is an open question of major interest beyond olfaction. To which degree there is a specific nuclear arrangement of DNA and how this is established after cell division would then be further important for other unsolved questions in biology.
— Tobias Ruff
-  Buck, L. and Axel, R. , A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 1991, 65-1 PP175-187 DOI:10.1016/0092-8674(91)90418-x
-  Hanchate, N. K. and Kondoh, K. and Lu, Z. and Kuang, D. and Ye, X. and Qiu, X. and Pachter, L. and Trapnell, C. and Buck, L. B. , Science 2015, 350-6265 PP1251–1255
-  Clowney, E. J. and LeGros, M. A. and Mosley, C. P. and Clowney, F. G. and Markenskoff-Papadimitriou, E. C. and Myllys, M. and Barnea, G. and Larabell, C. A. and Lomvardas, S., Cell 2012, 151-4 PP724–737
-  Markenscoff-Papadimitriou, E. and Allen, W. E. and Colquitt, B. M. and Goh, T. and Murphy, K. K. and Monahan, K. and Mosley, C. P. and Ahituv, N. and Lomvardas, S., Cell 2014, 159-3 PP543–557
Curious things happen around us all the time – and sometimes we are so familiar with them that we do not even notice them anymore.
If you read the title you might now think that this article was about the Leidenfrost effect , that is, this little funny dance water droplets perform on a hot surface such as a frying pan. It is not, though. The Leidenfrost effect occurs when a material – usually a liquid – meets a surface far above its boiling temperature. A thin layer of the droplet’s surface will then evaporate rapidly, causing a protective gas coating to appear that effectively insulates the droplet and lets it last longer on the hot surface. Similar effects can also be seen with liquid nitrogen on a material at room temperature. These droplets appear to travel around due to ejected gasses. But does a similar phenomenon also occur without the necessity of a hot surface?
There is in fact a location where such an effect occurs regularly without us usually noticing: The bathroom. Under certain conditions water droplets can be seen moving on a surface of water as if they had hydrophobic properties. The easiest way to see them is in the shower, when the shower floor is already covered in a thin layer of water. If new water droplets now impact on this surface at certain angles and speeds, they can be seen rushing around for a while before disappearing. It turns out that in recent years a few scientific publications were dedicated to investigating this effect more closely. [2,3] With a high-speed camera, the bouncing effect can be visualized rather easily, as shown in Fig. 1: The droplet appears to cause a dent in the water surface and then bounce off without merging with the rest of the liquid. Of course, the first idea that comes into mind now is the Leidenfrost effect, where a similar behavior can be seen caused by a layer of vapor. However, here no high temperatures are involved and thus the generation of water vapor is negligible.
The first intuition of an air coating to protect the water droplet is still standing, though, and thus the scientists tried to model the behavior. It turns out that there is indeed a protective coating of air, which can get compressed when the droplet approaches the surface of the liquid underneath. The air simply cannot escape quickly enough and therefore protects the droplet on impact and pushes away from the water surface. This phenomenon causes what is called the residence time of a droplet, that is, the time a droplet can sit on top of a pool of the same liquid before coalescing (see Fig. 2). The theory was confirmed by lowering the ambient air pressure around the experiment, which caused the residence time to decrease.  However, one would expect that this thin layer of gas should not withstand a heavy impact of a droplet coming from e.g. the shower head with a lot of speed and thus kinetic energy.
An explanation can be found using a simple speaker membrane: When the droplets are put in contact with an oscillation surface, like water on an oscillating speaker, the bouncing is facilitated, and the droplets can remain intact for much longer. Moreover, the droplets now travel around just like they do in a shower! High-speed camera footage can show the reason for this change in behavior: The surface of the water pool, excited into periodic up- and down-movement patterns, gently catches the droplet if the surface is moving downwards in the moment of impact and therefore prevents the impact from destroying the protective gas layer. It is just like gently catching a water balloon with your hand by grabbing it in motion and then slowing it down. Additionally, the continuous movement of the surface seems to stabilize the gas layer and therefore massively increases the residence time, all while allowing the droplet to travel from minimum to minimum, thus creating the “walking water” effect.  In a shower, the impact of many, many droplets cause the surface of the water pool on the ground to oscillate in a similar manner, creating landing spots for some droplets that then move around the surface. The phenomenon can thus be explained by the residence time of a droplet together with an oscillating surface.
Finally, one can reproduce a similar behavior in space, where microgravity does not pull the droplets down. An air bubble inside of a water bubble can thus act like an isolated system where droplets can form and move… excited by the sound of a cello! If you got curious, please check out the beautiful footage in Ref.  where much of the inspiration of this article came from.
As stated initially, the most curious things happen around us and we simply have to notice them.
— Kai Litzius
 Y. Couder et al., From Bouncing to Floating: Noncoalescence of Drops on a Fluid Bath, Phys. Rev. Lett. 94, 177801 (2005).
 J. Molácek & J. W. M. Bush, Drops bouncing on a vibrating bath, J. Fluid Mech. 727, 582-611 (2013).
 I. Klyuzhin et al., Persisting Water Droplets on Water Surfaces, J. Phys. Chem. B 114, 14020-14027 (2010).
 https://www.youtube.com/watch?v=KJDEsAy9RyM (Water bubble in space at time index 8:18).
Superstitions are having hard times in our modern always progressing world. It is no longer easy to fool someone with a myth or a beautiful legend from childhood. But how about this one: have you ever heard that a thunderstorm could curdle milk?
A correlation between thunderstorms and the souring or curdling of milk has been observed for centuries. As early as in 1685 the first clue was written down in the book “The Paradoxal Discourses of F. M. Van Helmont: Concerning the Macrocosm and Microcosm, Or the Greater and Lesser World, and Their Union” :
“Now that the Thunder hath its peculiar working, may be partly perceived from hence, that at the time when it thunders, Beer, Milk, &c. turn sower in the Cellars … the Thunder doth everywhere introduce corruption and putrefaction”.
By the beginning of the 19th century there had been numerous attempts to find theories of a causal relationship. [2-7] They all were not plausible, many even contradicting. Later, after refrigeration and pasteurization became widespread, eliminating bacteria growth, interest in this phenomenon almost disappeared. While the most popular explanation remains that these occasions are only a correlation, we would like to draw the reader’s attention to some of the suggested theories.
In order to understand what actually happens with milk during a thunderstorm we would need to know (i) what processes are behind the milk souring and (ii) what accompanies thunderstorm, e.g. lightning. While the latter is not yet entirely clear to scientists,  the simplified picture of the first point we will cover in the next few paragraphs.
Fresh milk is a textbook example of colloid – a solution consisting of fat and protein molecules, mainly casein, floating in a water-based fluid.  The structure of milk is schematically illustrated in Fig. 1. Fat globules are coated with protein and charged phospholipids. Such a formation protects the fat from being quickly digested by bacteria, which also exist in milk. Casein proteins under normal conditions are negatively charged and repel each other so that these formations naturally distribute evenly through the liquid. Normally, milk is slightly acidic (pH ca. 6.4-6.8),  being sweet at the same time due to lactose, one of the other carbohydrates within the milk. When the acidity increases to pH lower than 4, proteins denature and are no longer charged. Thus, they bind to each other or coagulate into the clumps known as curds. The watery liquid that remains is called whey.
The acidity of milk is determined by the bacteria which produce lactic acid. The acids lower the pH of milk so the proteins can clump together. The bacteria living in milk naturally produce lactic acid as they digest lactose so they can grow and reproduce. This occurs for raw milk as well as for pasteurized milk. Refrigerating milk slows the growth of bacteria. Similarly, warm milk accommodates bacteria thrive and also increases the rate of the clumping reaction.
Now, we can think of a few things that may speed up the souring process. The first one could be ozone that is formed during a thunderstorm. In one of the works it was shown that a sufficient amount of ozone is generated at such times to coagulate milk by direct oxidation and a consequent production of lactic acids.  However, if this were the case, a similar effect would occur for sterilized milk. The corresponding studies were carried out by A. L. Treadwell, reporting that, indeed, the action of oxygen or oxygen and ozone coagulated milk faster Ref. . But the effect was not observed if the milk had been sterilized. The conclusion drawn from this study was that the souring was produced by unusually rapid growth of bacteria in an oxygen rich environment.
In the meantime, a number of other investigations suggested that a rapid souring of milk was most likely due to the atmosphere that is well known to become sultry or hot just prior to a thunderstorm. This warm condition of the air is very favourable for the development of lactic acid in the milk. [3, 4] Thus, these studies were also in favour of thunderstorms affecting the bacteria.
A fundamentally different explanation was tested by e.g. A. Chizhevsky in Ref. . It was suggested that the electric fields with particular characteristics produced during thunderstorms could stimulate a souring process. To check this hypothesis the coagulation of milk was studied under the influence of electric discharges of different strength. Importantly, in these experiments the electric pulses were kept short to eliminate any thermal phenomena. Eventually, the observed coagulation for certain parameter ranges was explained by breaking of protein-colloid system in milk due to the influence of the electric field.
Other experiments investigating the effect of electricity on the coagulation process in milk turned out to be astonishing.  When an electric current was passed directly through milk in a container, in all the test variations, the level of acidity rose less quickly in the ‘electrified’ milk samples compared with the ‘control’ sample. Which contradicted all the previous reports.
To conclude, while there is no established theory explaining why milk turns sour during thunderstorms, we cannot disregard numerous occasions of this curious phenomenon.  What scientists definitely know is that milk goes sour due to bacteria – bacilli acidi lactici – which produce lactic acid. These bacteria are known to be fairly inactive at low temperatures. Which is why having a fridge is very convenient for milk-lovers. However, when the temperature rises, the bacteria multiply with increasing rapidity until at ca. 50°C it becomes too hot for them to survive. Thus, in pre-refrigerator days, when this phenomenon was most popular, in thundery weather with its anomalous conditions the milk would often go off within a short time after being opened. Independently of the exact mechanism, i.e. increased bacteria activity or breaking of the protein-colloid system, the result is – curdled milk.
Should you ever witness this phenomenon yourself, do not be sad immediately. Try adding a bit brown sugar into your fresh milk curds…
— Mariia Filianina
 F. M. van Helmont Franciscus “The Paradoxal Discourses of F. M. Van Helmont, Concerning the Macrocosm And Microcosm, Or The Greater and Lesser World, And their Union” set down in writing by J.B. and now published, London, 1685.
 A. L. Treadwell, “The Souring of Milk During Thunder-Storms” Science Vol. XVIII, No. 425, 178 (1891).
 “Lightning and Milk”, Scientific American 13, 40, 315 (1858). doi:10.1038/scientificamerican06121858-315
 H. McClure, “Thunder and Sour Milk.” British Medical Journal vol. 2, 651 (1890).
V. V. Fedynskii (Ed.), “The earth in the universe” (orig. “Zemlya vo vselnnoi”), Moscow 1964, Translated from Russian by the Israel Program for Scientific Translations in 1968.
 W. G. Duffield and J. A. Murray, “Milk and Electrical Discharges”, Journal of the Röntgen Society 10(38), 9 (1914). doi:10.1459/jrs.194.0004
 “Influence of Thunderstorms on Milk” The Creamery and Milk Plant Monthly 11, 40 (1922).
 K. Litzius, “How does a lightning bolt find its target?” Journal of Unsolved Questions 9(2) (2019).
 R. Jost (Ed.), “Milk and Dairy Products.” In Ullmann’s Encyclopedia of Industrial Chemistry (2007). doi: 10.1002/14356007.a16_589.pub3