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Why do your knuckles crack? Engineers say they finally figured it out

The pop of a good knuckle crack fills a room, but what actually makes the noise? Scientists have struggled to find a clear answer because it all happens so fast.

So researchers turned to mathematical formulas. They created equations describing what goes on in the fluid-filled space between the finger bones and the hand bones—the suspected source of the cracking. Then they tested a prominent idea to explain the sound: Bubbles form in the fluid-filled space when the joint expands during the act of knuckle-cracking, and some scientists think that the subsequent collapse of those bubbles is what causes the popping sound.

And indeed, the collapse of the “mathematical” bubbles in the knuckle joint generated sound waves that look just like the ones created in real life, the team reports today in Scientific Reports. This contradicts the findings of a 2015 study, which argued that it’s the formation of bubbles that creates the popping noise, not their collapse. It’s not clear whether the latest science will settle the debate once and for all—but at any rate, the next time you hear knuckles crack you can thank (or blame) the bubbles.