Cracking your knuckles?

• The sound you hear when you crack your knuckles is actually the sound of nitrogen gas bubbles bursting.
• The type of joints that you can most easily "pop" or "crack" are the diarthrodial joints. 
• The easiest joints to pop are the ones in your fingers (the interphalangeal and the metacarpophalangeal joints).
• But how can releasing such a small quantity of gas cause so much noise? There is no good answer for this question. Researchers have estimated the energy levels of the sound by using accelerometers to measure the vibrations caused during joint popping. The amounts of energy involved are very small, on the order of 0.1 milli-joule per cubic millimetre. Studies have also shown that there are two sound peaks during knuckle cracking, but the causes of these peaks are unknown.