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.