Hair gets its color when two types of pigment called melanin combine. Different proportions create a variety of shades. We don't yet understand the mechanics behind it, but hair follicles the 100,000 or so pits on our heads that sprout our hairs produce less melanin as we age. The result is gray hair.
While we could probably point a finger at stress, the timing of this color change is mostly a function of the genes you inherited from your parents. Stress can definitely trigger a higher rate of hair loss, though, so if you're no spring chicken and your hair follicles are producing less pigment, chances are the new strands that grow in will be gray and you could be sporting a salt-and-pepper look before you know it.
OK. So to summarize on gray hair, one, blame your parents and two, try not to stress out.
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Critical thinking challenge: Why does it take 100,000 hair follicles to cover our heads?