What is the cracking sound in your knuckles?

The cracking sound in your knuckles is caused by a rapid change in pressure within the synovial fluid that surrounds the joints. When you pull or stretch a joint, the capsule surrounding it is stretched, and the synovial fluid within it is put under negative pressure. This causes the dissolved gases in the fluid to come out of solution and form bubbles. When the joint is suddenly released, the pressure normalizes, and the bubbles collapse, creating a popping or cracking sound. This sound is often referred to as crepitus.