Quite a few people are able to bend their fingers, neck, back, wrist or any other join to make a cracking sound. But how does this phenomenon, that symbolises violence and brings a satisfying feeling, come about?
A joint (for the purpose of simplicity, this will only refer to synovial joints, or diarthrosis) is formed from two bones, covered by articular cartilage, being linked by an articular capsule (lined with synovial membrane) and ligaments. The joint space is filled with synovial fluid, which is a lubricant with a smaller coefficient of fraction than any man-made lubricant.
The synovial fluid is the substance that is most involved in the process of cracking a joint. It has carbon dioxide and nitrogen dissolved in it, which leaves the fluid if the pressure inside the joint falls, such as when it is stretched or bent. This causes a bubble to form, which quickly snaps the ligament surrounding the joint, making a whipping sound. Also, the bubbles then collapse as the pressure rises again, adding a popping sound. These two combined are what makes joints crack. It is also why there is a refractory period where the joints do not crack again for a while, as the gases must be redissolved in the synovial fluid.
Children are taught not to crack their fingers because it will thicken their joints and deform them. However, no research data supports this and instead states that it is a myth. Dr Donald Unger received an Ig Nobel prize (an award given for strange discoveries and triumphs in ten different areas) for cracking the joints in only his left hand and not the right for 50 years. He too found that the negative effects were minimal and had no substantial effect on arthritis.