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Have you ever stayed in a pool or bathtub so long that your fingers got wrinkly? This is normal — and can even affect your toes. But why does it happen?

Even though you can't see it, your skin is covered with its own special oil called sebum (say: SEE-bum). Sebum is found on the outermost layer of skin. Sebum moistens, or lubricates (say: LOO-bruh-kates), and protects your skin. It also makes your skin a bit waterproof. That's why water runs off your skin when you wash your hands, instead of soaking it in like a sponge would.

But staying in water for a long time washes away the sebum. Then, the water can penetrate the outer layer of your skin. This causes your skin to become waterlogged. So how does this lead to wrinkles? For a long time, people thought the water caused skin to swell up and get puffy. Now researchers believe wrinkly fingers could be an autonomic nervous system reaction. Why? Because it's easier to pick up wet objects with wrinkly fingers. Wrinkles on your fingers may give you more grip, kind of like treads on a car tire.

What should you do if this happens to you? Nothing. It goes away quickly on its own. You'll have more sebum on your skin in no time.