By Alexandra Sifferlin
April 16, 2015
TIME Health
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Scientists have answered the puzzling question of why our knuckles make that “pop” sound when we crack them.

A team of University of Alberta researchers had a volunteer crack his knuckles inside an MRI scanner so the researchers could figure out what was going on. They published their findings on Wednesday in the journal PLOS ONE.

The researchers have concluded that the crack comes from a gas-filled cavity or “bubble” that forms in the fluid between the joint.

“It’s a little bit like forming a vacuum,” said lead study author Greg Kawchuk, a professor in the Faculty of Rehabilitation Medicine 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.”

MORE: You Asked: Is Cracking Your Knuckles Bad?

The study supports an original theory from the 1940s, the researchers say. But in the 1970s, other researchers believed that the sound came from a bubble collapsing in the joint instead.

But is cracking your knuckles bad for you? That little bubble appears to be benign; there’s no evidence to suggest that people who crack their knuckles are more likely to suffer from arthritis than those who don’t.

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