system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some people complain that their bones hurt when it rains?

Usually It's people with old injuries. I've had broken bones, but I feel fine when the weather changes.
gpt
Because the barometric pressure in the air drops, and the water in your bones can't equalize fast enough, they swell, and cause general achy-ness.  

Old injuries probably hurt more because when you break a bone, and it heals, it always heals a little more than it needs to to make it stronger in that area.  So, there is more bone density in those people's bones than in normal people's bones, which means more swelling, and more achy-ness.