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Your joints can make a variety of sounds: popping, cracking, grinding, and snapping. The joints that "crack" are the knuckles, knees, ankles, back, and neck. There are different reasons why these joints "sound off".

Escaping gases: Scientists explain that synovial fluid present in your joints acts as a lubricant. The fluid contains the gases oxygen, nitrogen, and carbon dioxide. When you pop or crack a joint, you stretch the joint capsule. Gas is rapidly released, which forms bubbles. In order to crack the same knuckle again, you have to wait until the gases return to the synovial fluid.

Movement of joints, tendons and ligaments: When a joint moves, the tendon’s position changes and moves slightly out of place. You may hear a snapping sound as the tendon returns to its original position. In addition, your ligaments may tighten as you move your joints. This commonly occurs in your knee or ankle, and can make a cracking sound.

Rough surfaces: Arthritic joints make sounds caused by the loss of smooth cartilage and the roughness of the joint surface.

Is joint cracking harmful? If you are feeling pain when your joints pop, than you should seek a health care professional. In terms of knuckle cracking, some studies show that knuckle cracking does not cause serious harm. Other studies show that repetitive knuckle cracking can do some damage to the soft tissue of the joint. It may also lead to a weak grip and a swelling hand.

i think our bones cracks because arms or legs are paining.

Just trapped air/gas. You hear it when it's released suddenly.