Uncle John knows pretty much everything—and if he doesn’t, he heads his massive research library, or puts one of his many associates on the case. So go ahead: In the comments below, ask Uncle John anything. (And if we answer your question sometime, we’ll send you a free book!)

What’s the purpose of fingernails and toenails?

The purpose of our fingers and toes is fairly self-evident: the better differentiation of digits makes for better dexterity. This allows us to grasp small objects, or climb and walk, respectively. But why do we have these hard little disks on top of all of our digits? Surely there has to be more of a reason beyond painting them so we can show off our “nail art” on Instagram.

Fingernails and toenails actually assist in the what the digits themselves are there for: dexterity. Nails are what set primates—including humans—apart from other mammals. While most other mammals have claws they use to grab onto things, or climb, or dig, primates over time have lost their claws in favor of broad, individual fingertips. The nails have flattened out according to their use: to aid in the grasping of small branches, or picking fruit off of trees, for example, or, in the case of early human, handling small tools.

Nails also serve as an uncanny external warning system for internal problems that would otherwise go dangerously unnoticed. Oil spots or small depressions on nails is a sign of psoriasis, for example. Brownish spots may be a sign of AIDS or melanoma. Discoloration indicates malnutrition.

One more thing about nails: they’re made of keratin, the same material that hair is made of. Also made from keratin are horse hooves. Nails don’t have nerve endings, which is why it doesn’t hurt when you bite them.