What do you get when you combine the "Wayne Gretzky of knuckle cracking" with a magnetic resonance imaging (MRI) machine? The answer to a very old question, it turns out.

By using MRI to video-record knuckle cracking in action, researchers have discovered that the unsettling "pop" made by cracking one's knuckles results from the rapid creation of a cavity in the fluid inside the joints.

"It's a little bit like forming a vacuum," study researcher Greg Kawchuk, a professor of rehabilitation medicine at the University of Alberta in Canada, said in a statement. "As the joint surfaces suddenly separate, there is no more fluid available to fill the increasing joint volume, so a cavity is created, and that event is what's associated with the sound." [See MRI Video of Knuckle Cracking in Real-Time]

To discover this tidbit of anatomical weirdness, Kawchuk and his colleagues dreamed up what they call the "pull my finger" study. Chiropractor Jerome Fryer, of Nanaimo, Canada, got the ball rolling when he approached Kawchuck with a new hypothesis explaining why knuckle cracking makes a popping sound. Over the decades, researchers have speculated that pulling apart the finger joints creates or collapses bubbles in the joint fluid, and that perhaps one or the other could explain the noise.