The old solution is that wrinkling is simply the result of your fingers and toes absorbing water after a long period of being submerged. But there are problems with this! First: why is it only our fingers and toes that get wrinkly? Second: why is this such an unusual trait among mammals (only humans and macaques get wrinkly)? Third: why, if this is a simple tale of osmosis, do our fingers and toes cease to wrinkle when nerves to them are cut?