Wrinkly fingers and toes.

Everyone's experienced this: After taking a bath or a long shower, our fingers and toes get wrinkled.

What causes this? One word: osmosis. Osmosis is the movement of water across a membrane from high concentration to low concentration. In this case there is a higher concentration of water in the bathtub than in your skin.

So then why is it that only our fingers and toes get wrinkly, but not our arms or legs? This is because our skin has a layer of waterproof keratin on the surface, preventing both water loss and uptake. On the hands and feet, especially the toes and fingers, this layer of keratin is continually worn away by friction. Water can then penetrate these cells by osmosis and cause them to become wrinkled.