Why is it that your fingers get all wrinkly when you’re in the bath too long?
It’s a pretty simple little answer. You know how a spongue gets bigger when it gets wet. The outer layer of our skin is like that too — it soaks up a bunch of water and gets swollen. But it can’t just get big and puffy because it’s firmly fastened to the layer of skin underneath. But that extra surface area has to go somewhere, so it buckles up into folds, and wrinkles. This happens after a long time in the bath because the skin oils (sebum), which usually protect your skin, eventually washes away, letting the water in to your skin.
As a commenter on the Wonderquest site put it,
My high school biology teacher explained it as: you have a size 3 finger and size 3 skin. After you have been in water, you still have a size 3 finger but now you have size 7 skin.
This is all in the epidermis, the outermost layer of our skin. The stratum corneum is the part that is on the outside, and it’s got a bunch of dead keratin cells. Keratin’s the stuff found in our hair and fingernails. The dead keratin cells absorb water. It happens mostly on our hands and feet because those parts of our body go through a lot of wear and tear, so they’ve got more dead keratin cells on them than, say, the sensitive and soft underside of our arm.
Great physiology and physics!