THE NAME'S BOND, BROKEN BOND
A cotton t-shirt is made of cloth -- made of woven together threads, which are in turn made up of cotton fibers. These fibers are constructed of long molecular chains, which are linked end-to-end by hydrogen bonds. As the fibers are spun into thread and the thread is woven into cloth, the fibers and the polymers they're made of get pulled, stretched and twisted and the hydrogen bonds holding everything together get stressed.
This stress is relieved when you throw the shirt in the wash. The energy in the heat and agitation that clothes are exposed to during washing and drying break the stressed out bonds and the polymers are free to relax and return to their natural, pre-stretching size. In turn, the fibers, the threads and the cloth get smaller and you wind up with a shirt just big enough to fit the cat (come here, Mittens, it's time to play dress up!)
Pull out a piece of your hair (c'mon, it'll be fun). Pull it through your thumb and pointer finger by the root end, and then again from the opposite end. It doesn't feel as smooth the second way, right? That's because the hair is covered with small raised scales that are arranged sort of like shingles on a roof, all pointing in the same direction. Wool fibers have these scales too, and processing these fibers and making a wool sweater disrupts the natural fiber alignment and allows the scales to snag one another. The heat, water and agitation that meet the sweater in the washer and dryer basically ratchet the