Why do we have hair? The simple answer is protection. I've heard dermatologists argue about whether the scalp hair we modern humans have is enough to protect us from the elements. I have proof that it is: My bald grandfather's head gets cold when the temperature drops below 50° F (10° C), and his scalp and neck are covered with skin-cancer scars.

What hair's made of

You may have no interest whatsoever in knowing what the strands of your hair are made of, or how they grow. But the answers are interesting - and important, too. Once you understand how hair is built and how it functions, you'll find it easier to care for, be better able to pinpoint bogus hair-care claims, and know enough about how your locks behave to stop worrying about them and let them do what nature intended.

The only areas of the human body without hair follicles are the lips, the palms of the hands, and the soles of the feet.

Hair is an appendage of the skin. Each individual strand grows from a single live follicle that is anchored in the subcutaneous layer of your skin. A sebaceous gland that produces sebum is attached to each follicle. The oily sebum keeps the hair strand moisturized, shiny, and protected against toxic substances. Not everyone produces the same amount of sebum. The more sebum an individual produces, the oilier his or her hair is; the less sebum an individual produces, the drier his or her hair is.

Getting down to the roots

Before we go any further, let's return to the follicles, which contain the bulb-shaped roots of your hairs. These are nourished by tiny capillaries that carry minerals, proteins, vitamins, fats, and carbohydrates. It is here, at the root, that newly dividing cells force older cells upward, where they die and harden into the hair shaft.

The hair shaft has three layers: the cuticle, the cortex, and the medulla. If you look closely at the cuticle under a microscope, you'll see that it is made up of layers of colorless, tightly overlapping tiles, much like a tile roof. The cuticle is the strongest part of the hair fiber and protects the fragile inner cortex. The cortex is comprised of a a protein called keratin. It is also the place where melanin - your hair's natural pigment - is located. The medulla - which for an unexplainable reason some people's hair happens not to have - does not extend all the way to the hair's tip. Though the medulla contains a small amount of pigment, it is essentially colorless; it does, however, help reflect the light that travels through the translucent cuticle and cortex.

Depending on your hair's natural color, it contains one or two different types of melanin: Eumalin is the most common and is responsible for darker hair shades, including chestnut, coffee, and black, while phaeomelanin contributes to light and reddish tones such as blonde, caramel, ginger, and auburn.