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Cracking Joints

You may not be a knuckle cracker yourself, but chances are you have heard someone doing it. This noise is caused by pockets of gas that are escaping from the joints in the hand. There is something called synovial fluid that lubricates joints and contains nitrogen, oxygen, and carbon dioxide gas. When the joint is put in an abnormal position, bubbles of these gasses are rapidly released which causes the cracking noise. The reason a knuckle can rarely be cracked twice is because it takes time for the gases to build up again. Some claim that cracking knuckles leads to arthritis, but the studies claim that it only causes minor damage to soft joint tissue. A similar cracking noise can be produced with the knee or the ankle but it is not due to escaping gas. Tendons can be pushed out of place as joints move, and when they return to their original position they can create cracking noises. Because gas does not have to build up like it does in the knuckles, this noise can often be made repeatedly.

Source: everydaymysteries

What about the neck?