Summer's warm waters let you extend your soak time, and that means your fingers will wrinkle accordingly. Here's why. An oil called sebum provides a water-resistant coating for your skin. When you swim, the oil washes off, which allows lake water--thanks to osmosis--to penetrate the outermost layer of skin, causing it to swell. The skin puckers because it swells unevenly--more swelling where the outer layer is not attached to the second layer of skin, less swelling where it is attached.