The chemical is syn-propanethial-S-oxide! The compound was identified in 2002. The reaction occurs when cell walls are broken (by cutting). The chemical is volatile (turns into gas) and rises toward your eyes. Tears are the eye's natural defense mechanism. Tips to avoid crying: chill the onion (slows the reaction); use a very sharp knife (less cell damage); cut under running water (the chemical dissolves); wear goggles; cut near a vent (draws gas away). The "onion smell" is caused by other compounds (thiosulfinates). Onions are alliums (same family as garlic, leeks, chives). The pungency varies: yellow onions are most pungent; sweet onions (like Vidalia) are milder because they have lower sulfur content. Cooking inactivates the enzyme, which is why cooked onions do not make you cry. The LFS (lachrymatory factor synthase) gene has been identified; researchers have developed "tearless" onions (Sunions) using conventional breeding, not genetic modification. The tearless onion was introduced in 2018. A related phenomenon: wasabi and horseradish cause a burning sensation in the nose, not tears (allyl isothiocyanate). The tear response is a defense mechanism to deter animals from eating the onion (the gas irritates eyes, causing the animal to stop feeding).

Steam pressure causes the pop! The hull must be intact (unbroken) to hold pressure. When the pressure reaches about 135 psi (9.3 atm), the hull ruptures. The starch gelatinizes (turns into a foam) and cools into the fluffy shape we recognize. Unpopped kernels (called "old maids") are usually too dry (water content too low) or have a cracked hull that allowed steam to escape. The world record for the largest popcorn ball is over 3,000 pounds. Microwave popcorn uses a special bag that absorbs microwaves and creates a hot environment. Popcorn is a type of flint corn (Zea mays everta). Other types of corn (sweet corn, field corn) do not pop because their hulls are not strong enough or their water content is wrong. Popcorn has been consumed for thousands of years (ancient popcorn was found in Peru dating to 4700 BCE). The first commercial popcorn machine was invented by Charles Cretors in 1885. Popcorn became popular during the Great Depression because it was inexpensive. Movie theaters sell popcorn because it has a high profit margin. Air-popped popcorn is the healthiest; microwave popcorn often contains unhealthy fats and artificial flavors. The "pop" sound is caused by the sudden release of pressurized steam. High-speed video shows that popcorn kernels actually "jump" when they pop. Popcorn is a whole grain (rich in fiber). The hull is the pericarp (outer layer).

Ice is less dense than liquid water! Most substances are denser as solids (they sink). Water is unusual because of hydrogen bonding. When water freezes, the molecules arrange into a hexagonal lattice with open spaces. This structure is about 9% less dense than liquid water. If ice were denser, it would sink, and lakes and oceans would freeze from the bottom up, making it impossible for aquatic life to survive winter. Ice floating insulates the water below, preventing complete freezing. This property is essential for life on Earth. The lattice structure also explains why ice expands (water pipes burst in winter). The density of water is maximum at 4°C (about 1.000 g/cm³). Below 4°C, water expands as it cools (anomalous expansion). The floating of ice also affects climate: sea ice reflects sunlight, cooling the planet; if ice sank, the Earth would be much colder. The hexagonal symmetry of ice crystals creates snowflakes with sixfold symmetry. There are at least 18 different crystalline forms of ice (Ice Ih is the common form). Pressurized ice (Ice II, III, etc.) has different structures. The floating of ice is also why icebergs have only about 10% of their mass above water (the rest is below). Hydrogen bonding also gives water its high surface tension, high specific heat, and its ability to dissolve many substances. Without hydrogen bonding, water would boil at about -80°C. The unique properties of water make it essential for life. The water molecule is polar (positive on the hydrogen side, negative on the oxygen side), which creates hydrogen bonds between molecules.

Rayleigh scattering is the reason! John William Strutt (Lord Rayleigh) explained it in 1871. The intensity of scattering is inversely proportional to the fourth power of the wavelength (λ⁴). Blue light (wavelength ~470 nm) is scattered about 16 times more than red light (wavelength ~650 nm). On the Moon (no atmosphere), the sky appears black even in sunlight. On Mars, the sky appears pink-red because of iron-rich dust. Water droplets (much larger than molecules) scatter all wavelengths equally, making clouds white (Mie scattering). The sky on a clear day is not perfectly blue; it has a gradation from deeper blue overhead to paler near the horizon. The human eye is more sensitive to blue than violet, so we see blue. The sky can appear purple/violet if you have a camera with a special filter (the human eye is less sensitive to violet). The color of the sky affects plant growth (plants use red and blue light for photosynthesis). Rayleigh scattering also explains why sunsets are red: sunlight passes through a longer path of atmosphere, scattering away most blue light. Volcanic eruptions can inject particles that cause vivid sunsets (more scattering). The Tyndall effect is scattering by particles larger than molecules (but smaller than wavelength). The blue color of eyes is also due to Rayleigh scattering (not blue pigment). The sky is blue during the day, but at night we see stars. Astronauts see a black sky in space. The sky on Earth would be white if the atmosphere were thicker (like on Venus, but Venus's atmosphere is mostly carbon dioxide with thick clouds).

Pruney fingers are caused by the nervous system! For a long time, people thought it was just due to osmosis (water entering skin cells). In 1935, doctors noticed that patients with nerve damage did not get pruney fingers. The wrinkling is an active process controlled by the sympathetic nervous system. The evolutionary advantage: wrinkled fingers improve grip on wet surfaces (acting like rain treads on tires). A 2013 study showed that people with pruney fingers could grip wet objects faster. The effect is reversible (skin returns to normal after drying). The same wrinkling occurs on toes. Pruney fingers usually appear after about 5 minutes in warm water. Cold water slows the process. The effect is less pronounced in older adults. The term "pruney" comes from the resemblance to prunes (dried plums). Some people call it "aqua-wrinkling." The skin on palms and soles is thicker than elsewhere, which may be why wrinkling occurs there. Soaking for too long can cause maceration (breakdown of skin). The "waterproof" barrier of skin is provided by the stratum corneum (outer layer of dead skin cells). The pruney effect is temporary and harmless. Wearing gloves while washing dishes prevents the effect. The sensory nerves in fingers are involved in the reflex. The chemical aquaporins (water channels) may also play a role. If you have pruney fingers without soaking, it could indicate a medical condition (like dehydration or thyroid disease).

Brain cooling is a leading theory! Studies show that yawning frequency decreases when a cold pack is placed on the forehead and increases when a warm pack is placed. Other theories: yawning increases oxygen intake (but this has been disproven because oxygen levels do not change). Yawning may also stretch the lungs and increase heart rate, promoting alertness. Contagious yawning occurs in about 50% of adults and begins around age 4-5. It is less common in young children and older adults. People with schizophrenia and autism are less likely to catch yawns (linked to empathy deficits). Animals that yawn contagiously include chimpanzees, bonobos, dogs (catch yawns from humans), wolves, elephants, and budgerigars (parakeets). The average yawn lasts about 6 seconds. Yawning is often associated with boredom, but people yawn during exercise and before exams (when they need to be alert). Yawning is present in fetuses (as early as 11 weeks gestation). Some people yawn when they see someone yawn in a video. The exact neural mechanism involves the hypothalamus and brainstem. The neurotransmitter dopamine is involved (dopamine agonists cause yawning). There is no known purpose of yawning, but it is universal among vertebrates. The Guinness World Record for the longest yawn is not a category (because who would measure that?). Yawning is a classic example of a common behavior that science does not fully explain. Contagious yawning is related to theory of mind (understanding that others have mental states). Some people yawn when they think about yawning (you might be yawning right now!).

Arrector pili muscles cause goosebumps! The reflex is controlled by the sympathetic nervous system (same as fight-or-flight response). In animals with thick fur, erect hairs trap insulating air (in cold) or make the animal appear larger (to predators). In humans, the reflex is vestigial (leftover from evolution). We still get goosebumps when listening to music, watching a moving scene, or feeling awe. The term "goosebumps" comes from the resemblance to the skin of a plucked goose. The medical term is "cutis anserina" (Latin for "goose skin"). Other triggers: strong emotions (fear, excitement, nostalgia), fever, and certain drugs (opioids). Goosebumps are also associated with piloerection (hair standing on end). Some people can voluntarily give themselves goosebumps (via intense concentration or recalling an emotional memory). Goosebumps accompany the "chills" (frisson) from music. Frisson is a psychophysiological response to music (shivers down the spine). The frequency of goosebumps varies among individuals. Goosebumps on the arms are most common; they also occur on the back of the neck, legs, and scalp. Goosebumps do not serve a useful purpose in modern humans, which is why they are considered vestigial. The hair follicles of the arms have the most visible arrector pili. The reflex may also be triggered by certain medications (opioids). Goosebumps are sometimes called "chicken skin." In reptiles and birds, raising scales or feathers serves similar purposes. The arrector pili muscles are smooth muscles (not under voluntary control).

Physics explains it! The toast rotates as it falls. Table height and the toast's rotational speed determine the angle of impact. For a typical table (~2.5 ft / 0.75 m), the toast has time to rotate about 180 degrees. If the toast starts with the butter side up, it will land butter side down. If the toast starts butter side down, it will land butter side up (but people usually drop toast with butter side up). The effect is not random; it is deterministic based on height and angular velocity. A higher table would allow the toast to rotate multiple times, making the outcome random. A lower table would cause the toast to land butter side up. The experimenter's bias: we remember when toast lands butter side down because it is more frustrating. The butter side is heavier? Actually, butter does not significantly affect the center of mass. The phenomenon is sometimes called the "buttered cat paradox" (joke: you can't have a perpetual motion machine). A 2001 study using toast and a high-speed camera confirmed the 180-degree rotation. The toast would need to be dropped from a height of about 8 feet (2.4 m) to land butter side up consistently. Some breakfast tables are lower; this might affect results. The toast's shape (rectangular) affects rotation (different moments of inertia). The phenomenon is a good example of Murphy's Law in action. The buttered toast problem has been studied as a physics demonstration. The best way to avoid it: hold the toast with the butter side facing you, not up. The "butterfly effect" is not relevant. The toast's rotation is due to the torque applied when it slides off the table edge (not from falling straight down). The edge of the table provides a pivot point. A slice of toast that is simply dropped (not sliding) may not rotate. So the phenomenon depends on how it falls off the table (usually, it slides).

Water vapor is in the bubbles! The bubbles are not air; they are steam (gaseous water). The boiling point decreases with altitude (at high altitudes, water boils at a lower temperature). The bubbles form at nucleation sites (scratches on the pot, impurities in the water). Superheating can occur in very smooth containers (microwaved water can explode). The "singing" of a kettle before boiling is caused by bubbles collapsing. At the start of heating, dissolved air comes out of solution, forming small bubbles that collapse, producing sound. Bubbles in carbonated drinks contain carbon dioxide (not water vapor). The difference: boiling is a phase change; carbonation is dissolved gas. Bubbles in a simmering pot are smaller and less frequent; rolling boil bubbles are large and vigorous. Adding salt raises the boiling point slightly (but not enough to affect cooking time significantly). The Leidenfrost effect occurs when a droplet of water floats on a layer of steam above a hot surface (dancing droplet). The vapor bubbles can cause "bumping" when superheated liquid suddenly erupts. Bumping can be prevented by adding boiling chips (porous material that provides nucleation sites). In zero gravity (space), boiling bubbles do not rise; they coalesce into a single giant bubble. Boiling water is used to sterilize medical equipment (killing pathogens). The boiling point of water is 100°C at 1 atm (sea level). In Denver (5,280 ft / 1,609 m), water boils at about 95°C (203°F). This affects cooking times (pasta takes longer to cook at high altitudes). Pressure cookers raise the boiling point, reducing cooking time. The bubbles in a boiling pot rise because steam is less dense than liquid water. The "bubble" we see is actually a cavity of water vapor surrounded by liquid water.

Water expands when it freezes! This is due to the hydrogen bonding that forms a crystalline lattice with more space between molecules. Most substances contract when they freeze. The expansion of freezing water is why pipes burst in winter. The expansion pressure can exceed 2,000 psi (pounds per square inch) – far more than a soda can can hold. A soda can is designed to hold carbonation pressure (about 30-50 psi), not freezing pressure. If you accidentally freeze a soda can, let it thaw completely in the sink before opening (it may still be safe). The best way to quickly cool a soda is to put it in an ice bath (not the freezer). The expansion of freezing water also creates frost heaves in roads and cracks in rocks (freeze-thaw weathering). Beer cans also explode when frozen (alcohol lowers the freezing point, but beer is mostly water). Diet soda freezes at a slightly higher temperature than regular soda (because sugar lowers the freezing point). The expansion can split the seam of the can. Aluminum cans are not designed to withstand internal pressure from ice. Frozen soda cans can be dangerous (sharp edges). The carbon dioxide in soda is dissolved under pressure; when the can is opened, the pressure drops and CO₂ comes out of solution, creating bubbles. When soda freezes, the CO₂ is forced out of solution and can form gas pockets. The order of liquids that expand when frozen: water is unique among common liquids. Some liquid crystals and certain alloys also expand when solidified. Silica-based liquids (like liquid glass) also expand. The expansion of water is why ice floats. The same property allowed life to evolve in water (ice insulates the water below). Without this property, lakes and oceans would freeze solid in winter. The freezing point of water is 0°C (32°F). Adding salt lowers the freezing point (which is why salt is used on icy roads). The freezing point depression is a colligative property.