Wrapped around your body like a protective overcoat, your skin forms a barrier between the inside of your body and the outside world. Skin is waterproof, keeps out germs, and repairs itself. It filters out harmful rays in sunlight, gives you the sense of touch, and helps to control your body temperature.
Your skin is just millimeters thick, yet it makes up the largest organ in the body, accounting for about 16 percent of your weight. Its tough outer surface is designed to wear away, so it continually renews itself from below.
Skin also produces hair and nails. Like the outer surface of skin, these protective tissues are made of dead cells hardened by a tough protein called keratin.
Skin consists of two layers. The top layer - the epidermis - is mostly dead and provides protection. The epidermis is paper-thin on the eyelids but about a quarter of an inch (5 mm) thick on the soles of your feet. Under the epidermis is the dermis, a living layer riddled with blood vessels and nerves that can sense touch and pain.