Ask a grown-up: why do old people have grey hair?
Actually, lots of old people don't have grey hair, either because they colour it with brilliant modern methods, or they're lucky (like me) to have limited grey hair.
However, here's the technical stuff in answer to your question: old people have grey hair because of the process that controls production of a pigment called melanin – the same thing that makes your skin tan when you are out in the sun. This is what gives hair different colours, such as brown, blond or red. Each hair follicle (the tissue that surrounds each strand of hair) contains a certain number of pigment cells and, as we get older, they gradually begin to die. Because there are fewer pigment cells to produce melanin, hair starts to change colour, and the grey starts to come through.
As people continue to get older, eventually the hair will become grey all over – something that I hope doesn't happen to me for an extremely long time.
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