Why do clothes shrink in the wash, and how do you prevent it?
Heat releases tension locked into fibers during manufacturing. Cold water and air drying prevent nearly all of it.
Fibers - especially cotton and wool - get stretched during spinning, knitting, and finishing. Hot water and dryer heat let them relax back to their natural length, and the garment shrinks. Wool adds a second mechanism: its scales ratchet together under heat and agitation (felting), which is irreversible. Prevention is boring and total: cold water, gentle cycle, and air dry for anything you care about. Pre-shrunk labels ('sanforized') mean the mill already did the relaxation for you, and such garments shrink less than 3%. If something does shrink, a soak in lukewarm water with hair conditioner, then gentle stretching back to shape, rescues cotton and wool surprisingly often.