According to the Library of Congress, bones crack because of the combination of escaping gases, movement of the joint and rough surfaces. Not all joints make cracking sounds. Those joints that make audible sounds include the knees, ankles, knuckles, back and neck.
The Library of Congress explains that joints contain a lubricating liquid called synovial fluid. Synovial fluid contains several dissolved gases, including oxygen, nitrogen and carbon dioxide. When a joint is bent or pulled apart, the gases in the synovial fluid escape. This is what causes the sounds made when a person cracks his back. The reason that a joint cannot be cracked repeatedly is that it takes time for the gases to replenish in the synovial fluids.
In addition to the escape of synovial fluid, arthritis causes joints to make sounds. This occurs because many arthritic joints have missing or worn-out cartilage. This makes the bones grind against each other, producing cracking or grinding noises. Tendons also cause sounds to come from a joint, according to the Library of Congress. In some joints, especially those of the knees and ankles, tendons snap back and forth, which can create a cracking sound. Medical professionals disagree as to whether repeatedly cracking joints is harmful.