Long-standing mystery solved at last!

Long-standing mystery solved at last!  What makes the sound when you crack your knuckles?  It's a bubble of gas forming.

Fun knuckle-cracking facts...  It's not just your finger joints that can be cracked – some people can crack their toes, too.  And did you know that there are (at least) three different parts of your finger joints that can be cracked?  Some people can crack all three on the same fingers...