The bones themselves aren't cracking. There's a certain amount of gas that's held in suspension in the fluid in a joint and when you stretch the joint the gas goes from a liquid state to a gaseous state and you'll hear a pop or crack.
It's not unlike opening a beer. You can't see the bubbles when the bottle is unopened because the pressure keeps the gas in suspension. But as soon as you open the bottle, the bubbles come out of suspension, make the foam dribble overtop of the bottle and ruin your coffee table.
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