system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do dogs go grey around the mouth? Do animals have "beards"?
gpt
The same reason many humans generally start going gray at the temples first and then it spreads to other parts. It's because as we and the animals age, certain pigment-producing cells at the root of our hair die, and that usually starts in specific body regions first. 

Normally as the hair grows, those cells inject a colour into the hair, but when they eventually die off in some body regions as most people (and dogs) age, they're not there any more to inject the dye, so the growing hair becomes essentially fingernail-clipping coloured.  

Genetics determines when and where this happens, in the same way that it determines other hair features such as waviness or patches of colour on a cat's or cow's hide (except that's not as time-related).