If your hair drier shrunk your head to the size of a grapefruit, you'd be astounded. If your wool pants shrank in the drier, you'd be angry but not amazed. How can clothing disobey the laws of common sense, and shrink in the laundry?
To understand why this happens, we need to take a closer look at the microscopic structure of natural fibers like wool. Each thready strand you can see is composed of millions of long, tangled molecules called "polymer chains." Microscopically, these start out coiled like miniature, tangled springs, something like a mass of tangled telephone cords. These curly molecules are stretched and straightened as the wool is carded and spun.
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. Instead, 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.