The release of gases in the most common cause of the sound you hear when you crack your back.
In most of your body's joints, there is a trace amount of synovial fluid that acts as a lubricant. This fluid contains gases like nitrogen, carbon dioxide, and oxygen.
The presence of these gases are responsible for the popping noises you hear what you crack your back or your knuckles.
When you crack a joint, you are stretching the capsule and causing the gas to be released at a rapid pace.
This is also the reason you cannot immediately re-crack that same joint. You must wait until the gases are replenished in that capsule of synovial fluid to allow them to crack and pop once again.
Other common causes of a snapping, cracking, grinding, or popping joints include moving tendons and ligaments.
Occasionally when a joint is moved, a tendon will change positions and move out of place. When it returns to its normal position, you may hear a pop or snap.
In older adults, osteoarthritis is a probable cause of grinding in the joints.
Arthritic joints that are lacking in smooth cartilage have a tendency to 'grind' against each other when moved due to the roughness of the joint's surface.