The unmistakable sound of a knuckle cracking is caused by a gas cavity forming between finger joints, according to a new study.

The cavity, or bubble, forms in the synovial fluid, the lubricant between joints. The discovery was reported in the current issue of the journal PLOS One.

One of the paper’s authors, Jerome Fryer, a chiropractor in British Columbia, lay inside a cine magnetic resonance imaging machine, which stitches together video from a series of rapid scans. Researchers at the University of Alberta cracked each of his knuckles by pulling on a cable attached to his fingers.

“You’ll see the black cavity that occurs just as the cracking occurs” on the video, said Dr. Greg Kawchuk, a professor of rehabilitation medicine at the university and the lead author.