Rubber has two surprising habits: stretch it and it warms up, and warm it while it is stretched and it pulls itself shorter.
In 1805 John Gough, a blind English scientist, described both effects after feeling them with his lips, and in the 1850s James Joule measured them. The explanation comes from counting the ways a chain can lie.
Many ways to coil
A rubber chain is always wriggling, jostled by the heat of its surroundings, and every wriggle moves it into a new arrangement. A chain can curl up in vastly more ways than it can lie straight. So the jostling almost always leaves it coiled, for the same reason that shuffling a pack of cards almost always mixes it up.
Chemists measure how many arrangements a state can have with a quantity called entropy. Left to themselves, jiggling molecules move toward the state with the most arrangements, the highest entropy. A stretched band pulls back because its chains, wriggling at random, keep falling back into coils.
Where the warmth comes from
When you stretch a band, your pull does work on it. A steel spring stores that work in its stretched bonds. Rubber's chains uncoil while their bonds keep their lengths, so the work goes into the jiggling of the atoms, and faster jiggling is a higher temperature: the band warms up. When a stretched band pulls your hands back together, it does work on them, and that energy comes out of the jiggling of its atoms, so the band cools.
Warm rubber pulls harder
Hang a weight from a rubber band, warm the band with a hairdryer, and the weight rises. Hotter chains wriggle harder, so they pull back toward their coils more strongly, and the band shortens. A metal wire with a weight on it gets a little longer when it is heated, as most materials do.