A down jacket is insulated with the soft and warm under feathers of ducks or geese. Down is an excellent insulator because its loft (or fluffiness) generates billions of small air pockets that trap warm air and hold heat, keeping the user exceptionally comfortable in cold winter temperatures. The term "duck down" came about when women would sew their own clothes; they would use the down from inside out-dressed birds to make themselves look more attractive and to keep warm. Down is made up of feathers from a variety of sources including geese, ducks, and other birds. When harvested by hunters, this source material provides only a temporary solution for insulation because it gets wet through quickly when exposed to rain or snow. Down can also wear out over time due to mechanical stress caused by wearing layers of clothing. The primary ingredient in goose down is collagen, which gives skin, bones, ligaments, and tendons their strength. There are two main types of collagen: type I, which is found in young animals, and type II, which becomes more abundant as people age. Both types of collagen are useful for making clothing, but because type I goes out of style so quickly it's not practical to harvest much of it. Goose down is widely used in high-end down jackets because of its unparalleled warmth-to-weight ratio.