When you crack your knuckles, gas is released. Where does this gas come from, and more importantly where does it “go” when cracked?
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This bubble process is called [cavitation](https://en.m.wikipedia.org/wiki/Cavitation). Basically when you suddenly extend the space inside the joint by stretching it, the same amount of synovial fluid takes up less space, and the pressure inside the joint drops; as that happens the gases (mostly nitrogen and hydrogen) that were dissolved into the joint fluid can no longer stay dissolved, so they spontaneously turn into bubbles. As the knuckle comes back, the pressure goes up, the bobble pops, and you get noise. At the end of which, the gases are back in an environment which has enough pressure, so over the next few minutes they dissolve back into the fluid. So it’s almost entirely a closed system, the gases were always there, and they go back afterwards.
As a fun aside, cavitation is *very* important in submarines. Propellers of ships and submarines reduce the pressure of water just forward of the propeller, and if they’re going fast enough, the propeller drops low enough for air bubbles to form in the same way they do in your knuckles. When they go pop, they make a very loud and very detectable noise. If you’re a sub trying to stay silent and hidden underwater, this is very bad. So submarines have to go slow when they don’t want to be heard by other ships; billions of dollars gets spent in designing propellers that have less cavitation.
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