Knuckle cracking - what is actually happening to the joint?

"Our jaws were on the floor. It was like, 'Wow! we can't believe what we're seeing!'" says Kawchuck.

What they first saw was a white flash. Kawchuck thinks that's the synovial fluid that lubricates the joints. Then as the bones pulled apart from that finger tug, "magically, we saw this black cavity form at the same time that the sound was produced."

It was a gas bubble, and it stayed there after the crack.

Read on ...