Each strand of hair has a thin tube of tissue, called a follicle, at its root under the skin of your scalp. Each follicle contains pigment cells which produce melanin - the same pigment that makes your skin darker or lighter. The amount of melanin in your hair follicles will determine how dark or light your hair is too - the more melanin you have, the darker your hair or skin.

As you get older, the pigment cells begin to die off. The less melanin they produce, the more transparent the hair colour will be - that is, the more grey or white. When all the melanin producing cells have died, the hair will turn completely white.

Some people's pigment cells die very early, usually for hereditary reasons but sometimes in response to stress or a shock. It is often though that dark-haired people go grey earlier, but in fact this only seems to be the case because dark hair shows grey more than light hair.