Joints produce a cracking sound when nitrogen bubbles burst in a thick and clear fluid surrounding the joints to lubricate them, called ‘synovial fluid’. When we stretch or twist our fingers, the pressure in the fluid decreases, which leads to dissolving of gases present there, forming bubbles, through a process called “cavitation”. When the joint is stretched far enough, the pressure drops and the bubble burst, producing a pop sound. It takes about 25-30 minutes for the gas to redissolve into the joint fluid. During this period, the knuckles won’t crack.
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