Sweaters shrink in the wash primarily due to the structure of wool fibers and the effects of heat and agitation, which can cause felting and tension relaxation. When wool is processed for sweaters, the fibers are stretched and smoothed out, leading to a different behavior when exposed to water and heat compared to the natural state of wool on sheep. Sheep's wool contains lanolin, a natural oil that provides water resistance and helps maintain the integrity of the fibers, preventing them from tangling and shrinking when wet. The curly and kinky nature of sheep's wool also means that when it gets wet, the fibers tend to curl inward rather than shrink outward, unlike the fibers in a sweater, which can shrink in all directions. This difference in fiber structure and the presence of lanolin are key factors in why sheep do not shrink when caught in the rain, while sweaters do when washed.