What's Happening When You Crack Your Knuckles?

Synovial fluid occupies the cavities of synovial joints, and serves as a lubricant for joint movement. The fluid is full of dissolved gases that form microscopic bubbles when the space inside the joint is stretched. An April 2015 study determined that the formation of these bubbles—not their collapse—is what produces the familiar popping sound.

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Written by Curiosity Staff January 16, 2015