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from Kidsworld.com
What's really happening when you "crack" your knuckles is that 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 (made mostly of 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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