Why do fingers get wrinkly in the bath?

Fingers wrinkle in bath water because your nervous system tells tiny blood vessels in your fingertips to narrow. That pulls the skin underneath inward, while the outer skin has extra room, so it folds into ridges. It is an active body response, not simple soaking.

Those ridges may help wet fingers hold slippery things, a little like grooves on a shoe sole. The same trick happens on toes, where grip matters too. Once your child dries off, the vessels loosen again and the smooth fingertip shape returns.