Skin

"The skin is the largest organ of the body, with a total area of about 20 square feet. The skin protects us from microbes and the elements, helps regulate body temperature, and permits the sensations of touch, heat, and cold.
Skin has three layers: The epidermis, the outermost layer of skin, provides a waterproof barrier and creates our skin tone.
The dermis, beneath the epidermis, contains tough connective tissue, hair follicles, and sweat glands.
The deeper subcutaneous tissue (hypodermis) is made of fat and connective tissue.
The skin’s color is created by special cells called melanocytes, which produce the pigment melanin. Melanocytes are located in the epidermis."

Frostbite

Frostbite is, literally, frozen body tissue — usually the skin, but sometimes deeper tissue — and must be handled carefully to prevent permanent tissue damage. The varying degrees of frostbite are based on the depth of the tissue injury, ranging from a superficial area of the skin all the way down to the muscle and bone in the most severe cases.