There are different reasons depending on what your clothes are made of. When we wear our clothes we stretch all the little fibers that they are made of, and putting them in hot water helps them “remember” their original shape. That’s why most clothes shrink a little bit after washing. Clothes made from plant fibers like cotton will do this.
Some animal fibers, like wool, are covered in tiny little scales we can’t see. Your hair is like this—you can feel like scales by feeling the difference between pinching a strand and running your fingers away from your head (in the direction of the scales) vs. toward it (against the direction of the scales).
When you heat up the fibers the scales relax and lift up like little hooks. That alone won’t shrink your clothes much! It takes some agitating to do that. When the fibers get jostled around in the washing machine or dryer those hooks start grabbing on to each other like Velcro. The fibers get more and more tangled and closer together until eventually there’s no way you could untangle them. When the clothes cool down, the scales flatten back down while still hanging on to each other. Then the fibers are really locked together. This is why after you shrink wool it feels thicker and stronger than it did before.
We can remove the scales from the hairs with chemicals, and then they won’t shrink. But the fibers will be a little weaker as a result.
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