Skin

It's more than just a bag surrounding our pulpy bodies: the skin actively helps to keep us alive. The skin produces pigments that protect against the sun's ultraviolet rays. It also keeps our temperature constant. Subcutaneous fat holds in our warmth in cold weather, while sweat glands cool us when it becomes too hot. Each square inch of the skin contains 9 feet of blood vessels, 600 pain sensors and 134 yards or nerves. It also has 30 hairs, an outgrowth of skin that helps protect our bodies. Fingernails and toenails, too, are just a specialized kind of skin.