system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why head hair continues to grow, while pubic/leg/arm/underarm hair seems to reach a limit growth?
gpt
All hair has a limit on how long they can grow. Most people's hair on their head will generally stop growing when it's waist-length. Different hair have different length because they all have different length of life-cycle. Your eyelash has a life of 7-8 weeks, therefore it will only grow to a very short length before falling out. Hair on your head on the other hand, have a life of approx. 7 years, so those hair can be very long. Genetics can determine exactly how long your hair can grow, and some people's hair might have longer lifespan than others.