Coin flipping is a time-honored tradition for making decisions. Long before the NFL used the method to determine opening kickoffs, Romans employed coin tossing to settle personal disputes (though they called it “heads or ships,” a reference to the Roman coin’s two-faced Janus on one side and the prow of a ship on the other). While the mechanics of coin flipping are simple enough — guess a side and flip — the physics of how a coin flips are anything but. By exploring this complicated motion, scientists have discovered that coin flips are not as random (and thus impartial) as most of us think.

The Wright brothers decided who’d fly first with a coin toss.

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On December 14, 1903, Orville and Wilbur Wright flipped a coin to decide who’d be the first pilot of their eponymous flyer. Although Wilbur won, his luck didn’t last — his flight attempt failed, allowing Orville to soar into the history books three days later.

A 2023 study from the University of Amsterdam flipped 350,757 coins across 46 different currencies and discovered that a coin flipped to its starting position 50.8% of the time — close to 50/50, but not quite. In other words, if a coin started heads up, there was a slightly greater chance it would land heads up, too. This proves a previous theorem, developed in 2004, which argued that coin tosses landed as they started about 51% of the time. This small difference likely won’t dissuade humans from practicing the coin flip tradition, however. A more serious concern comes from a 2009 study, which revealed that coin tosses can be easily manipulated with just a few minutes of practice. So if you’re relying on the “randomness” of a coin toss to determine important decisions, make sure you trust the person doing the flipping.

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Sunlight tends to be good for us. It helps our bodies create vitamin D and mood-lifting serotonin, and even syncs our circadian rhythms. However, some people experience an unexpected side effect after glancing into the sun: sneezing. As many as one in four people have the reaction, appropriately called ACHOO syndrome (short for autosomal dominant compelling helio-ophthalmic outburst). The sun isn’t the only thing to blame — the reaction can occur when moving from dark to light settings, after seeing bright lights, or even from witnessing a camera flash.

It’s impossible to sneeze with your eyes open.

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Keeping your eyes open during a sneeze might feel uncomfortable — and difficult — but it can be done. However, some doctors suspect there’s a good reason our bodies reflexively close our eyelids: Doing so helps protect your eyes from the irritants being expelled from your nose.

ACHOO syndrome — also called “photic sneeze reflex” or “sun sneezing” — isn’t an allergy. While researchers aren’t entirely sure why it happens, one theory is that it’s caused by a nervous system misfire involving the trigeminal nerve, which connects the eyes and nose with the brain. Within seconds of seeing bright light, the pupils of the eyes contract and stimulate this nerve, possibly causing the nose to accidentally sneeze. People who experience ACHOO syndrome may get a runny nose and watery eyes, too, though these symptoms tend to disappear within a few minutes. Sun sneezing also has a genetic component; children of parents who have the photic sneeze reflex have a 50% chance of experiencing the same phenomenon. 

Some people diagnosed with ACHOO syndrome also reflexively sneeze when undergoing anesthesia, though for the most part the condition is more of an annoyance than a health concern. While there’s no treatment for sun sneezing, it is possible to reduce occurrences of the involuntary reaction with a few handy accessories, like hats and sunglasses, which block sudden bursts of light.

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Human ears have two main jobs: alerting us to noises in the world around us, and keeping our bodies balanced as we move throughout the day. However, researchers have found another small trick our ears can perform: determining the temperature of water. While they may look identical, hot and cold water make different sounds when poured. With a little help from our brains — which learn to recognize the contrasting tones over a lifetime of repeated exposure — most people can tell the difference between cold water poured from a pitcher or hot water transferred from a kettle, without even seeing the cup being filled.

Salt makes water boil faster.

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There is truth to this kitchen lore — adding salt makes it harder for water molecules to become steam, creates a higher boiling point, and makes water hotter, faster. But in small amounts, like a teaspoon, salt’s effects are limited, meaning this trick saves just a few seconds of time.

Without the cues of visible boiling or a collection of ice crystals, liquid water tends to look the same regardless of its temperature. What humans can’t see is that temperature actually affects water’s viscosity (aka thickness), which produces different sounds that we can hear. At lower temperatures, water molecules are sluggish and create temporary bonds that thicken the liquid, producing a lower-frequency sound when poured. Conversely, heated water molecules are more energetic, making it harder for molecules to join together. Fewer bonds result in thinner water, which produces a noticeably higher-pitched sound when transferred into a cup. Researchers say bubbling also plays a role in water acoustics — hot liquids have more bubbles than cool liquids, which contribute to the higher tones we hear while pouring out a cup of coffee or tea.

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