What causes the popping sound of your knuckles when you crack them? Why do some people feel the urge to crack them? Is there a buildup of something? And finally, can cracking your knuckles REALLY cause arthritus?
Thanks
Air. You stretch your ligaments when you crack your knuckles, and that’s not a good idea. It’s not as much fun as it is cracked up to be, but less cracked up than those idiots who persist in doing it.
Nitrogen. It’s suspended in the sinovial fluids that are used by the body as a lubricant in joints. Compressing the joint bones and therefore squeezing the fluid causes the bubbles to burst, releasing the gas and making the sounds.
I’m not a doctor, but I play one frequently in dark bars and nightclubs along Christopher Street where life is seamy and knuckles get raw 
Cartooniverse
Close but not quite.
Pulling or overbending the joints causes a lessening of pressure, causing the gas in solution in your sinovial fluids to appear as bubbles. When they get big enough, they pop.