system
Given the following post, respond as if you're commenting on Reddit:
human
What makes our hair and facial hair never stop growing when pubic and arm-pit hair stop at certain length?

Bonus question: How about animal furs?
gpt
Basically, there are different kinds of hair that grow--and more importantly for our question here--fall out at different rates.

Body hair falls out at about the same rate new body hair can grow to the same length to replace it so it tends to stay a "constant" length. Hair on the head falls out much slower than body hair, so it can grow much longer.

Fur on animals is typically the same as body hair on humans, though some animals--dogs and cats--go through rhythmic periods of hair loss called shedding.