Why does cracking your knuckles make that noise?

What's really happening when you "crack" your knuckles:

The noise you hear is actually the breaking of small bubbles of carbon dioxide. This is an absolutely harmless thing to do despite all the "old wife's tales" to the contrary. It is nevertheless a very annoying thing to do when it's very quiet unless of course that's what someone wants to do.

You are either pushing the joint back into or out of its normal position. Joints are the meeting points of two separate bones which are held together by connecting tissues and ligaments. A thick, clear lubricant (which contains some carbon dioxide and some nitrogen) called synovial fluid is found between the bones. When you stretch or pull your finger to get that desired popping noise, you are causing the bones to pull apart. Pressure is reduced on the synovial fluid and bubbles form that quickly expand and then burst - which is why you get that noise.

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