You know that familiar sound that comes from lacing your fingers together, turning your palms away from you and bending back your fingers? Joints produce that familiar CRACK when bubbles burst in fluid that surrounds the joint.
Joints are where two separate bones meet. They are held in place by connective tissues and ligaments. All the joints in our bodies are surrounded by a thick, clear liquid called synovial fluid. When you stretch or bend your fingers to pop the knuckles, you are causing the bones of the joint to pull apart. When they do, the connective tissue capsule surrounding the joint is stretched. Stretching this capsule causes its volume to increase. As we know from chemistry class, an increase in volume results in a pressure decrease. So when the synovial fluid pressure drops, gases dissolved in the fluid become less soluble. This forms bubbles through a process called cavitation. If the joint is stretched far enough, the pressure drops so low that these bubbles burst. That's when we hear the familiar pop we associate with knuckle cracking.
It takes about 30 minutes for the gas to re-dissolve into the joint's fluid. During this time, your knuckles will not crack. Once the gas is re-dissolved, cavitation is again possible. You can pop your knuckles again.