Explaining Our Body on Instagram: "what actually happens when you crack your knuckles. The popping sound does not come from bones grinding or joints being damaged. Instead, it’s caused by a process known as joint cavitation, which happens inside synovial joints like the knuckles, knees, and spine. Inside these joints is a thick, slippery fluid called synovial fluid that helps lubricate movement. When you pull or bend your fingers, the joint space temporarily increases, causing pressure inside th