As a chiropractor I have heard the popping noise that joints make and the process of popping a joint referred to in many different ways. Patients often ask, "Are you going to snap my back?" "Will you crack my neck?" "I think my back is out of alignment, I need you to put it back in." and so on. Are parts of the neck really snapping or cracking to cause that noise? What's really happening when you or your chiropractor pops your neck, back, or other joint?
The joints in your spine, knuckles, knees, etc. have fluid within the joint space that keeps it lubricated. When the joint is stretched the pressure within the joint space changes. If the joint is stretched far enough the pressure within it changes so much that gas is formed within the fluid filled space. This gas is then released from the joint, causing the "pop" noise.