Believe it or not, scientists have debated this question of wrinkly toes and fingers for quite some time. Most of the skin we see on our bodies is actually dead skin! Our fingers and toes have especially thick layers of this dead skin (also called keratin.) When we get in the bath tub or pool, those dead skin cells absorb water. Just like a dry sponge absorbing water, the skin grows slightly in size and it begins to wrinkle. The longer you stay in the water, the more wrinkled your toes and fingers get! By the way, warm water usually wrinkles skin faster than cold water.
It’s worth noting that new research points to our bodies actively wrinkling our fingers and toes to give us better grip. Some studies show that people that have neurological problems with their fingers or toes do not wrinkle up as much.