Find out how the cracking of knuckles works for the same reason deep sea divers have to take breaks as they ascend, soda fizzes, and water boils instantly in a vacuum.
We already know, thanks to a sixty-year-long experiment by a proud IgNobel Winner, that knuckle cracking does not cause any harm. Donald Unger cracked his knuckles twice a day - on his left hand. He let his right-hand knuckles remain uncracked. He did this for fifty years, since his mother warned him about knuckle-cracking as a child. At the end of those five decades, his left hand was in the same shape as his right. While doctors still advise people not to crack their knuckles, it's not so surprising that no damage is done. The snap we hear isn't something getting broken in our joints, it's something returning to its original state. And it's guided by a very powerful physical law.
Henry's Law states that, under a certain pressure, gas will dissolve in a fluid until it reaches a certain concentration. Change the pressure and you change the amount, and concentration, of gas in the liquid. Decrease the pressure and the gas comes bubbling out, like fizz from a soda bottle. And, in the end, we're all soda bottles, keeping our various fluids at various pressures so that gas can dissolve into it. We're living in a soda bottle, too, the pressure of which keeps the oceans from boiling away. The main reason more fluids don't fizz when they're coming out of us is we try to keep them at the same pressure as the atmosphere around us. That includes our blood, the reason why divers have to take breaks as they come up from the depths. At a higher pressure, such as the body would be under if it had many feet of sea water pressing down on it, more gas dissolves into the blood stream. The gas does nothing at first, moving around the body harmlessly until the diver moves up and out of the deep, when it turns the blood to foam and disrupts everything it comes into contact with.
If only blood were as isolated as synovial fluid, the stuff we have in our joints. This is the fluid that helps our bones slide against each other without grinding, and skin encase joints without folding into the spaces between the bones. It's kept in relatively well-sealed areas, and because of the constant tension that joints are subjected to, kept at a decent pressure. When we pull or stretch our joints, we relieve that pressure, and up come the bubbles of dissolved gas, flowing around the joint. As the joint slides back into place the pressure is restored and the bubble bursts with a despairing snap before the gas dissolves back into the fluid.
So that little snap doesn't make much of a difference to the hand, but it does make a difference to a physicist. This is, really, the beauty of physics. One law, one equation, can usefully explain why soda comes shooting out of a can, why knuckles crack, why the ocean doesn't boil, why deep sea divers need to take their time. From water bubbling in a vacuum to the inside of the human knee, it's all subject to the same, simple law. If we can be impressed that our bones are made up of disintegrated stars, we can be impressed that the gases bubbling through lakes of methane on Titan share the same physical patterns as the gases bubbling through our blood.