How Many Calories are on a Smudgy Screen?

Hey, Vsauce. Michael here. And I'm home for the holidays. I'm in my parents' basement, using a different camera than usual. But you know what is always different? Fingerprints. The palms of our hands and the soles of our feet are weird. They are covered with friction ridges, which act like the design of a stamp that stamps everything you touch.

Plastic or patent prints are visible to the naked eye. But even with seemingly clean hands on a seemingly clean surface, a fingerprint can be left. A latent fingerprint. But what are those made of? Well, by the square millimetre, they're made out of this. All of this stuff is natural. In fact, a lot of it is produced to keep your skin healthy, waterproof, pliable.

But touch a surface enough and the gunk can build up. It's what causes smudges on computer screens and mobile devices. Now, given that protein contains 4 calories per gram, fat contains 9 calories per gram and the surface area of the iPhone 5 is 4,400 square millimetres, completely licking clean a smudgy iPhone 5 screen is gonna give you about 2 calories. If you were to lick clean 245 smudgy iPhones 5, well, that would be the same as eating a Big Mac.

But before you run off and get all of your nutrition from smudgy phones, you should know that us humans need a bit more variety than that. I've always found it interesting that cats and dogs can grow up and live great lives eating the exact same food every day.

Well, what about us humans? What can we eat every day without ever changing it up and be fine? It turns out that there is one type of food that you could eat every day for the rest of your life and be fine, without ever having something different. If you follow @tweetsauce, you already know what it is. The answer is human breast milk.

Whether your ridges loop or arch or whorl is largely genetic. But the tiny minutiae of your prints formed while you were developing inside your mother. It tells a story about the very specific conditions around the dividing cells that became your skin. And for that reason, even identical twins, who pretty much share the exact same DNA, wind up with different fingerprints.

I could clone you 100 times and every one of those clones would have a unique set of fingerprints. Your skin has two major layers. The epidermis on the outside and the dermis, which is below. The basal layer of your epidermis creates new skin cells, which migrate up towards the outside world, where they die and flatten out, forming the protective outermost part of your skin - the 'horny layer' - which means that all day, every day, you are covered with and showing off to the entire world your horny layer.

These outer cells are constantly shed away, but their arrangement stays the same, because their shape is determined by the shape of the basal layer down below. This is why you can superficially and temporarily sandpaper or cut off your fingerprints, but they'll still grow back the way they were before.

But why do we have fingerprints in the first place? I mean, not all mammals have friction ridges. Is this some sort of cruel joke the universe made to make it more difficult for us humans to get away with crimes? Probably not. The most likely answer is two-fold. First of all, it has been shown that friction ridges allow us to sense subtle textures 100 times better than if we didn't have them.

And because fingerprints loop and arch and swirl in different directions, all it takes is one simple touch for us to detect a subtle texture moving in any direction. It's also been shown that having friction ridges allows mammals, like us, primates, koalas, the North American fisher, to better climb and grasp wet stuff.

Even though no two fingerprints are exactly the same, the methods we have for matching fingerprints aren't always perfect. There are plenty of different ways to model fingerprints and each one is calculated at different probabilities of a false match. But if you wanna commit a crime, it's best to just avoid fingerprints altogether.

How about hiring someone with adermatoglyphia? A few skin conditions can cause the disappearance of fingerprints. But adermatoglyphia is a genetic one that has no other side effects. But it's rare. Only four extended families on Earth are known to have the condition.

But if you wanna avoid leaving fingerprints and can't find somebody with adermatoglyphia, don't worry, just find someone who hasn't gone through puberty yet. Sure, kids might snot and slob all over stuff, but they carry far less heavy waxy oil on their skin.

And for this reason, the fingerprints of a child will disappear, evaporate, much more quickly than those of an adult. A child's fingerprints can evaporate in only a few hours. So, use kids to commit your crimes. And as always, thanks for watching.