Some parts of human skin, better known as glabrous skin, have a unique response to water. Unlike the rest of the body, the skin of our fingers, palms, and toes, and soles wrinkles after becoming sufficiently wet. Five minutes or so will usually do the trick.

But why do these patches of skin wrinkle? Some think that this is a biochemical reaction, an osmotic process in which water yanks a handful of compounds out of the skin, leaving an expanse of parched, pruney skin in their place.

But a century ago, scientists already knew that this curious reaction wasn’t a simple reflex or the result of osmosis.

That’s because surgeons learned that if certain nerves to the fingers were cut, the wrinkling response would disappear. Wrinkled fingers, then, are signs of an intact nervous system. Indeed, the wrinkling response has been suggested as a means of determining whether the sympathetic nervous system is functional in patients that are otherwise unresponsive.