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Cavitation bubbles! Which I did an episode of scishow on a while back. Cavitation bubbles form and then immediately implode when certain forces act on a liquid. They're a serious problem for boat propellers and hydraulic pumps because they can actually have quite a lot of energy behind them.

This same thing happens when you crack your joints. A tiny bubble of air in the fluid in your joints is snapping out of existence. When this happens, the energy that went into forming the bubble is released and we can hear it being released as a cracking sound.
Michael Aranda: What makes knuckles crack?

Hank Green: Cavitation bubbles! Which you can learn more about here: (SciShow episode on cavitation bubbles)  Cavitation bubbles form and then immediately implode when certain forces act on a liquid. They're a serious problem for boat propellers and hydraulic pumps. This same thing happens when you crack your joints. A tiny bubble of air in the fluid in your joints is snapping out of existence. When this happens, the energy that went into forming the bubble is released and we hear that energy as a snap.