[header] How to stop cracking your knuckles [title] Understand what causes the cracking sound. [step] When you crack your knuckles, you are manipulating certain joints in the body in a manner that promotes the release of gas (currently thought to be primarily nitrogen) from synovial fluid. Synovial fluid exists within synovial joints, and its function is to reduce friction between cartilage. 
When a knuckle is cracked, the gases dissolved within the synovial fluid are compressed and form of a bubble. This bubble then bursts, making that familiar popping sound as it does so.