How does hair know when to stop growing, e.g. eyebrows?

[whiskygalore]

Is it to do with genes/DNA? If so, why does some people's hair grow longer than others?

[paulobrad]

I think it has something to do with the muscles at the base of the hair, these can almost "sence" the hairs length and stop after a while. As to why some people grows longer than other I guess it is like why some people is curly or red, just genes.

[Gef]

Because they don't get it cut.

[punkmuppet]

It doesn't stop, it falls out and is replaced.