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A better question would be, "when does hair know when to fall off?"
Hair never stops growing.
Edit: when I say "hair," I mean one single strand of hair. That single strand of hair will eventually fall off. The thing is that not all strands fall off at the same time. So hair, the full head of hair, seems to be of the same length (especially if we keep getting haircuts.) But it's not like all hairs grow and then all of them collectively say "ok, everyone, let's stop growing!" and stop. No, each single strand of hair falls off, but at different times.
No, hair does stop growing.
Hair grows in phases and cycles. At the end of the cycle, it falls out.
The difference between body hair and the hair on your head is that the latter one has cycles measuring years, the other weeks.
This is the correct answer. Here's a little graphic on the phases.
This is unhelpfully pedantic given the OP's misconception.
Hair does not (appear to) stop getting longer because it stops growing. It (appears to) stop getting longer because older (longer) hairs fall out.
What’s „unhelpfully pedantic“ about a correct answer that explains OPs misconception? 🤡
The person above said hair doesn’t stop growing. That’s wrong. It does. It grows, then it stops growing, then the dead hair falls out. Why does it know when to fall out? Because it’s dead, Jim.
OPs question was why the hair on their head grows longer. Answer: because it’s growing cycles are longer.
I’d say you’re unhelpfully pedantic telling other people giving helpful and correct explanations they’re „unhelpfully pedantic“.
I’d say you’re extremely unhelpful because you give an „explanation“ that’s just complete bullshit and doesn’t explain anything.
I don't think we're disagreeing. That's exactly what I meant. But I can see how my wording could have been misinterpreted, so I'll edit it.