Hair is the collective term for thin, threadlike strands extending from a mammal's epidermis, the outer layer of the skin. Hair appears on mammals in varying degrees and serves many different purposes. Thick-haired mammals rely on hair for warmth. In human beings, hair around the eyes and inside the ear protects us by keeping foreign objects away. Most of the human body is covered by tiny hairs, except the palms of the hands and soles of the feet. Human hair growth begins in the embryo.

Each strand is composed chiefly of a protein known as keratin. Hair growth begins inside a soft sac or bulb where rapidly dividing hair cells multiply. Crowding inside the bulb pushes the old cells upward, forming the root and hair shaft. At the base of each hair bulb is a structure called a papilla, which houses the nerves and blood vessels necessary for hair growth. The root is protected by the hair follicle, a pouch of epidermal cells and connective tissues. The familiar "goose bumps" result when a tiny muscle attached to the hair follicle contracts.

The color and texture of hair are hereditary characteristics. Hair color largely depends on the amount and distribution of a brownish-black pigment called melanin. Hair also contains a blonder, or yellow-red, pigment. This lighter-colored pigment is most noticeable in those with little melanin. Red hair, however, contains a pigment all its own. As a person grows old, pigment stops forming and hair often turns gray or white.

Texture depends on the shape of each individual hair strand. Ironically, a round strand of hair results in straight hair. Wavy or curly-headed persons have flatter strands. Microscopically, the flattest hairs are the curliest.