Why do bones crack when you stretch or pop knuckles?

When you stretch or pop your knuckles, the cracking sound comes primarily from gas bubbles in the synovial fluid that lubricates your joints. This fluid contains dissolved gases like carbon dioxide and nitrogen. When you pull or bend a joint, you rapidly increase the space within the joint capsule, causing pressure to drop and gas to come out of solution, forming a bubble that collapses or pops, producing the sound. This is called cavitation. For other joints like knees or ankles, the sound can also come from tendons snapping over bony structures or rough cartilage surfaces moving against each other. After knuckle popping, it typically takes about 20 minutes for the gases to dissolve back into the fluid before you can crack them again.