When stress triggers a chemical called norepinephrine, it can permanently wipe out the stem cells that give hair its color, so once they're gone, your hair can't naturally regain its color.

Stress can turn your hair gray by messing with nerve signals in your hair follicles, which damages the cells that give hair its color — and once they're gone, the graying is permanent.

When mice experience sudden stress, it causes the cells that give hair its color to panic and leave their home in the hair follicle, burning out too soon and making the fur turn gray forever.

As we get older, the stem cells that keep our hair colored might stop working properly because of problems in their internal repair system, and once they're gone, our hair turns gray for good.

When the stem cells that give your hair its color get worn out or change too early because of stress in the cells, they stop making pigment, and your hair turns gray permanently.

When scientists temporarily stop the growth of pigment-producing stem cells in mice using special drugs or genes, the mice don't go gray under stress—because their pigment cells stay safely in place and keep coloring their fur.