system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do some hairs only grow to a certain length?

I've seen people with very long head hair but never anyone with braided armpit hair or flowing in the breeze pubes. Why?
gpt
Every hair is produced in a hair follicle. The follicle is an organ that produces hair in several stages. First the hair enters the growth stage called: "anagen". The hair follicle determines how long it will stay in the growth stage until it goes on to the next stage: resting. 

How long the hair keeps growing is determined by how long the hair follicle will stay in the growth stage. The hair follicle in your eyebrows typically stay in the growth stage for 2-3 months. But the hair on your head keeps growing for a few years until it goes on to the resting stage.