What causes that "Popping" Sound?

Some chiropractic adjusting techniques cause an audible noise that people refer to as the pop or crack. It is not a cracking or popping of bones. Chiropractors adjust vertebra of the spine. This sound is actually created by the change of pressure within the joint resulting in gas bubbles being released- it's exactly the same as when you "crack" your knuckles. It is not painful.