Why do your fingers wrinkle in water?

Sit in the bath too long and your fingertips turn into little raisins. Lots of people think the skin soaks up water like a sponge and puffs up. That sounds sensible. It is also wrong — and we have known it was wrong since 1935.

That year, two doctors, Lewis and Pickering, were examining patients whose median nerve was damaged. They noticed something odd. Anywhere the damaged nerve could not reach, the skin simply would not wrinkle. The skin was there. The water was there. Still no wrinkles. So the order to wrinkle does not come from water. It comes from the nerves.

Here is what really happens. Water slips in through the tiny sweat pores all over your fingers. That changes the balance of salts inside the skin. The nerve endings wrapped around the blood vessels just under the skin get excited and fire faster, telling those vessels to squeeze tight. When the vessels squeeze, fluid is pulled out of the skin. With less fluid holding it up, the skin sinks into folds. So it is not the skin puffing up — it is the skin sinking in.

Why wrinkle at all? One popular idea is that the folds work like tyre treads, giving you a better grip on wet things. A 2021 experiment found that wrinkled fingers needed about 20% less force to hold a wet object. But a 2014 team could not repeat that result, so the question is still open. Doctors do use the trick, though: if your fingers wrinkle in water, the nerves in your hand are probably fine.

Try it yourself: in the bath, put one hand in warm water and keep the other one out. Check every 5 minutes. The fingertips wrinkle first — their outer skin is thickest, and they have no hair or oil to keep the water out.