Bending a pencil eraser stretches the rubber on the outside of the bend and squashes the rubber on the inside. Both distortions cost energy. But the thinner the rubber, the less energy you need to stretch and squash. Bending becomes easier.
That thin elastic sheets bend so readily endows them with a way of accommodating stress that thick slabs lack. If you place two fingers on your arm and move them apart, your skin will respond like any other piece of elastic material, whether sheet or slab: It stretches. But if you move your fingers together, your skin, rather than compressing, will buckle out of plane: It wrinkles.
Despite wrinkling's ubiquity and seeming simplicity, the underlying mechanism is physically subtle and mathematically formidable (see the article by Michael Marder, Robert Deegan, and Eran Sharon, Physics Today, February 2007, page 33). The governing equations, derived in 1907 by August...
