August’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.

Jeff Smith is editor of Traverse, Northern Michigan’s