By the way, doctor: Why does skin wrinkle after being underwater?
Q. Why do our hands and feet wrinkle up when immersed in water? Your article about moisturizers said they work by trapping water in the skin, so why don't we wrinkle up after applying moisturizer?
A. The top layer of our skin, the stratum corneum, is like a sponge that absorbs water when it's immersed. And when it's filled with water, it expands and gets softer and more pliable. And, as you've noted, only the under sides of the hands and the feet prune up, especially the fingertips and the toes. That's because the stratum corneum is thick there.
Some people think that the wrinkles occur because of osmosis — water flowing out from the skin into a less "salty" environment. This mistakenly suggests that the wrinkles from the bath are like the tiny depressions and folds of dry skin. In reality, bathtub wrinkles are raised areas of skin puffed up by water absorption. One explanation that's been offered for pruny skin is that the absorbed water affects nerves, which leads to constricted blood vessels that pull the skin downward, creating furrows.