Researchers estimate that some 300 million people around the world are colorblind, most of them male. On the opposite end of the spectrum are those with an exceedingly rare genetic condition that allows them to see nearly 100 million colors — or 100 times as many as the rest of us. It’s called tetrachromacy, or “super vision,” and it’s the result of having four types of cone cells in the retina rather than the usual three. (Cones help our eyes detect light and are key to color vision.) Because of the way the condition is passed down via the X chromosome, the mutation occurs exclusively in women.

Dogs see in black and white.

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While humans have three cones to detect light and color, dogs have two. They have trouble seeing red and green, but researchers think they can make out shades of blue and yellow.

One tetrachromat describes her ability this way: “If you and I look at a leaf, I may see magenta running around the outside of the leaf or turquoise in certain parts where you would just see dark green. Where the light is making shadows on the walls, I’m seeing violets and lavenders and turquoise. You’re just seeing gray.” In short, tetrachromats see colors within colors, and even the tiniest change in the color balance of a particular hue will be apparent to them. It's estimated that 12% of women have a fourth retina cone, but only a fraction of them experience tetrachromacy. In total, only about 1% of humans have the condition. The rest of us will just have to close our eyes and imagine what it’s like. 

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You inherit half of your DNA from each parent, 17% to 34% of which comes from each grandparent for an average of 25%, and 12.5% from each great-grandparent. Beyond that, it gets murky — so much so that you aren’t genetically related to all your ancestors. 

The farther up the family tree you go, the more diluted your genetic link becomes; once you get past your great-great-great-grandparents, with whom you share about 3% of your DNA, the more likely it becomes that you aren’t genetically related to your relatives. For example, the percentage drops to a meager 1.56% with your fourth great-grandparents. If you ever see a picture of your great-great-great-great aunt and can’t detect a family resemblance, it may very well be because you didn’t actually inherit any of her DNA.

You inherit more genes from your mother than your father.

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Mitochondrial DNA comes from the egg, not the sperm, resulting in you inheriting slightly more genetic material from your mother than you do from your father.

The chance becomes greater with each generation you go back, of course. It increases from a 17.76% chance of not sharing any DNA with one of your sixth great-grandparents to a 37.43% chance with your seventh great-grandparents and a 57.53% chance with your eighth great-grandparents. The DNA you share with most of your cousins is also fairly meager: an average of 14.4% with first cousins, 3.4% with second cousins, and just 0.8% with third cousins.

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Most animals with eyes produce tears, but only one releases them in moments of sorrow and joy: humans. Creatures throughout the animal kingdom also feel emotion, but humans are the biological anomaly for being able to stream tears down our faces during times of emotional upheaval or relief — and scientists remain perplexed about exactly why. 

Babies are born without functioning tear ducts.

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Newborns cry a lot, and those early bouts of bawling are usually dry runs, leading some parents to believe their babies don’t have tear ducts. They do. At first the glands produce a scant amount of tears just for eye lubrication, but they increase to full-on waterworks in four to 12 weeks.

Unusual theories have cropped up over time: Aristotle believed the act of crying was simply to help clear our minds from suppressed feelings. Some have believed tears help cleanse the body of toxins. In the 1600s, it was common to think that our feelings heated the heart, which produced water vapor to cool itself that escaped through our eyes. Danish scientist Niels Stensen’s 1662 discovery of the lacrimal gland — located at the outer corner of each eye, and the origin point of our emotional tears — helped disprove that theory, though Stensen believed tears only moisturized our eyes. Even famed British naturalist Charles Darwin considered emotional tears to have no real benefit, though today’s scientists have a competing idea, believing that crying may provide a social cue to other humans that we need help; after all, babies cry an astounding amount and need round-the-clock attention.

Human eyes actually produce three types of tears, all with different purposes. Basal tears help to clean and lubricate the eyes, while reflex tears are a response to our environment (they appear when we cut onions). Emotional tears are the only kind we have some control over, and they have a slightly different chemical composition with more protein that may help them better stick to our faces.

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If you’ve ever seen someone track their pulse (in real life or on a crime drama), you’ll notice that the index and middle finger are always pressed on the neck’s carotid artery, which is responsible for transporting blood to the brain. There’s a reason why doctors (and actors who play doctors on TV) use these fingers and not, say, their thumbs. While your thumb is good for many things, taking your pulse isn’t one of them. Unlike the other four digits, the thumb has its own exclusive artery, the princeps pollicis, which makes it biologically unreliable as a pulse reader — because you’ll feel it pulse instead of the artery in your neck.

Today’s “thumbs up” gesture comes from ancient Rome.

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Today, “thumbs up” means liking things, whether YouTube videos or Facebook posts. But this positive gesture likely comes from WWI pilots, who used the hand signal to give the “OK” on preflight checks.

Among the 34 muscles, 29 bones, and three major nerves in the hand, there are also two key arteries supplying blood to the area: the ulnar and the radial. The ulnar artery branches at the wrist into a network of blood supply vessels called the superficial palmar arch, which then branches to supply blood to the top four fingers. The radial artery, meanwhile, branches at the wrist into the deep palmer arch, which then branches into the princeps pollicis artery, sending blood to the thumb. But today, there are more modern methods of tracking your pulse that use technology in lieu of touch. The Apple Watch, one of the most popular consumer fitness-tracking devices, relies on a process called “photoplethysmography,” which leverages the fact that blood reflects red light and absorbs green light. The watch uses green LED lights that flash hundreds of times per second, as well as light-sensitive photodiodes that help measure the amount of green-light absorption, and thus blood flow and pulse — no fingers (and definitely no thumbs) required.

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