why do your feet and fingers go wrinkly in the bath?

Answer 1

The water makes the skin swell, but as it's stretched pretty tight over your fingers and toes, it can't get big like a balloon. So it swells as much as it can, and it gets wrinkly because it's baggier than usual.

Like when elastic wears out in clothes--they get baggy and get wrinkledy instead of lying smooth. Same with your skin.