How do certain hairs on our bodies (for example, on legs) know what length to stop growing at?
This same question was posted recently and there were a ton of good answers. Maybe in /r/askscience ?
They don’t. Instead, hairs get damaged and shortened by the world around you over time. All hair experiences this. What makes hair a certain length is a combination of how fast/frequently it grows, and how thick and sturdy it grows. Remember, your hair is mostly keratin, like your nails. And like your nails, it can be ‘filed’ down by friction.
Also, your body regularly drops hairs it doesn’t want anymore. Eyelashes are possibly the most obvious example of this. They’re thick hairs, they grow relatively quickly, and they don’t get much wear and tear. But they also fall off a lot. Head hair is thinner, but also doesn’t get much wear and tear, and doesn’t fall off nearly as much, unless you’re balding.
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The cells in hair roots are not always active. When they are active, hair grows, and then the cells rest some time, which makes the hair “die” and drop. Then after some time they start growing a new hair. The length of the active time varies between different kinds of hair roots and the longer it is, the longer the hair.