system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do human stop growing? How does the body know it reached adulthood so no more growing indefinitly?
gpt
When we're born, our skeletons are made up of both bone, and this soft flexible material called cartilage. The soft cartilage is near the ends of the bones. Bone CAN grow (there are little cellular workmen who walk all over your bones breaking it down and building it up in places where the bone gets pulled on the most, which is why exercise is good for your bones) BUT not very much. We get taller, really "grow up," because the soft cartilage can grow fast.

Imagine a classic cartoon dogbone. In a baby, kid, and even most teenagers, the place where the straight long middle of the bone meets each the wide tips is made up of cartilage. This cartilage grows outward, and slowly turns into more bone. The more good foods and vitamins you eat, the more building blocks your body has to grow the cartilage and then make it into bone.

The older we get, the more of the soft parts get used up and turned into bone. Eventually, it's all firmed up and all of it is bone, and that's the tallest you'll ever be! Different bones finish hardening at different times: your head bones finish before your leg bones, and the last ones to finish are usually the collarbones that connect your shoulders to your chest. By the time a person is in their early to mid 20s, their skeleton is fully bone, not cartilage.

If you want to feel some cartilage, touch your nose or ears. That's what the stuff is inside, that feels not as hard as bone but not as soft as skin, fat, or muscle. That's why our noses and ears grow throughout our whole lives. The nose on a 70 year old will be bigger than it was when they were 30, even though their bones stopped growing in their early 20s!