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The Popping Sound Cracking Knuckles Make Continues To Intrigue

NOEL KING, HOST:

Good morning. I'm Noel King with the answer to a mystery that has puzzled scientists and kids for years. When we crack our knuckles, why do they make that popping sound? A grad student in France, Vineeth Suja, was cracking his knuckles in class and thought he could figure it out. He worked up a series of math equations, and, well, it's complicated, but there are little bubbles in your joint fluid. And when you crack your knuckles, the sound you hear is the bubbles popping. It's MORNING EDITION. Transcript provided by NPR, Copyright NPR.