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Many people like cracking their knuckles. I have always been curious about it. What happens when we crack knuckles?

What creates the POP sound?

Why do we get a relief-like feeling after doing it?

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Nobody really knows where it comes from. The currently most popular theory is that pulling the joint apart leads the gases in the joint's cartilage to accumulate and form a bubble which then pops when you let it spring back. The only thing that has been researched is whether it has an effect on the joint, but people who do it regularly don't seem to have any problems with their joints more than anyone else (I'll find the reference once I have time). Apart from that it's hard to get funding for this kind of research because it has little practical use.

Relief like feeling? That must be subjective to you, I used to do it every now and then but I don't remember getting any relief from it. In that case, it's probably the same relief as other people get from picking their nose, their ears, biting their nails, etc. It's called compensation reaction.

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anecdotal I know, but I also enjoy popping my knuckles. May be all in my mind, but I can 'feel' when I haven't for a while, and having done so there is a feeling of 'relief'. People have always told me that it increases your chances of arthritis - if you have a reference that says otherwise I'd much appreciate seeing it also. Thanks –  Luke Jun 1 '12 at 8:25
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Not on-topic enough to warrant an own answer but if you want to add the reference to the research about the harm, here’s an explanation and the link to the Ig Nobel prize research on Skeptics. –  Konrad Rudolph Jun 1 '12 at 10:40
    
It is possibly both boiling an cavitation, so will very likely lead to damage of body parts –  user959 Jun 4 '12 at 0:49
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