Requested by Age2B visitor
Joints can make different noises, some are serious and some are not.
Some people learn how to "pop their knuckles." By pushing or pulling a joint in a certain way, an air bubble can suddenly appear in the joint with a "pop." Once the bubble is there, the joint cannot be popped again until the air has been reabsorbed.
Snapping and popping of joints are common. The sound you hear is caused by air bubbles in the synovial fluid – the liquid that surrounds and lubricates your joints – and by the snapping of tightly stretched ligaments as they slide off one bony surface onto another. This sound is sometimes called crepitus – especially when you hear it in your knees going up and down stairs.
Some joints crack as the ligaments and tendons that pass over them slide past bumps on the bones. Individuals who "crack their neck" make noise in this way.
Other joints lock up intermittently, often with a loud pop, because something gets caught in between the joint surfaces. A torn cartilage in the knee or a loose piece of bone or cartilage in the joint can do this. Once a joint is stuck in this way, it may need to be wiggled around to unlock it. This may also cause a pop.
Finally, joints that are arthritic may crack and grind. These noises usually occur each time the joint is moved. This noise is due to the roughness of the joint surface due to loss of the smooth cartilage.