When you crack your knuckles, you're creating bubbles

The next time you crack your knuckle, know that you're actually creating little gas-filled bubbles in the fluid that lubricates your knuckle's joints. The cavities appear because the bones at the joints separate rapidly, creating a low-pressure volume that's filled by gas 'pumped' out of the higher-pressure fluid.

This is what Greg Kawchuk, a professor at the University of Alberta, Canada, and his colleagues discovered, by observing a participant crack knuckles under the gaze of an MRI scanner. Their findings are reported in a paper in PLOS ONE, published April 15. He attributed the noise specifically to the sudden formation of the cavity in the synovial fluid, "a little bit like forming a vacuum".

For its apparent simplicity, the study actually seeks to lay to rest the question of what causes the sound when knuckles are cracked. Since the early 1900s, multiple explanations have been advanced. All of them agreed that a cavity was involved in the cracking, but the primary contention was if the cracking sound was caused by a cavity forming or a cavity collapsing.

Kawchuk's use of the MRI rules in favor of cavity-formation. In fact, it also shows that the cavity persists well after the cracking is done, or as the paper puts it, "past the point of sound production". So the cracking couldn't have arisen from a collapsing cavity. Here's a film - the first one of knuckle-cracking - showing what the MRI revealed.