Does the sound of cracking knuckles make you flinch? What about a film of the joints in action? MRI scans have been used to capture the moment your joints go pop.
Gregory Kawchuk at the University of Alberta, Canada, and colleagues used a cable to slowly pull a man's fingers in an MRI scanner until the joints cracked. The sound was thought to come from the collapse of an air bubble, but in the scans the air cavity that formed in the fluid around the separating joints persisted after the noise.
A mysterious flash also appeared just before the crack. Kawchuk thinks it may be caused by cartilage releasing fluid as the tension on the joints rises.