Mouse pigment cells that darken after UV exposure can turn back into stem cells and keep making color for up to two years.

When mouse skin is exposed to UV light, pigment cells temporarily turn dark but later revert back to stem cells.

In mice with a specific genetic problem, hair turns gray because the stem cells that make hair color die off first—not because the color-making process itself is broken.

As we age, the stem cells that give color to our hair can start acting up — they might turn into pigment cells too soon or in the wrong place, which could be why our hair turns gray.