In your pants, the straightened polymer chains have a natural tendency to spring back to their original, tightly-coiled state. Fortunately for clothing manufacturers, it’s not exactly easy for the natural fibers to do this. In order to curl back up, they need a certain amount of energy. It’s hard to picture this, because we’re used to springs bouncing back to their original shape as soon as we take the tension off them. These microscopic springs don’t work that way though–they stay stretched out until they get a boost of energy to help them scrunch back up. Running your pants through a hot drier, or washing them with hot water, can give them the energy they need to spring back. When this microscopic scrunching up happens all over the fabric, your pants shrink.