Why and how do some hairs “know” when to stop growing (e.g. eyebrows, arms, eyelashes) while others (e.g. scalp, beard) doesn’t seem to?

Question

Is it that they don’t know or the “threshold” is so long that we don’t get to see it very often?

In: Biology

Answer 1

They don’t stop growing, they simply fall out. Stronger hairs last longer, and therefore can grow longer (like head hair). Eyebrow hairs for example fall out much more regularly, even though you likely don’t notice them.

Source: have some ultra strong eyebrow hairs (specifically 3) that I have to cut. The rest are normies