Stress makes you go gray, we've all known that forever. But we never knew why. Until now.

Scientists at Harvard have finally figured it out. It has to do with your sympathetic nervous system, which is responsible for your fight or flight response under stress.

It turns out that when it gets activated, it can produce a chemical that attacks the cells that color your hair.

And when it happens enough times, your hair goes gray for good.

So, now you know.