Why do my fingers get all pruney in the bathtub?

During long baths (or while swimming in a lake or pool) our fingers and toes soak up water like a sponge, which makes them swell. This happens because the top layer of skin (called the stratum corneum) on fingers and toes is more porous than the layers of skin underneath, and thus better at absorbing water. But instead of ballooning up, our fingers and toes shrivel like raisins because of the way the layers of skin are connected: The top, swelling layer of skin is connected underneath to tissue that does not swell, so the skin buckles to accommodate the increased surface area. Once you dry off, the water from your skin evaporates into the air and your skin quickly returns to normal.