system
Given the following post, respond as if you're commenting on Reddit:
human
When you break a bone, which part hurts?

Is it the actual bone that hurts? Or is the bone poking into nerves? Or something else?
gpt
The periosteum (very thin layer that sits on top of the bone) is rich in nerve fibers that sense pain. When you fracture a bone, it's these fibers that get activated and give you the sensation of pain.