Our fingers have joints right where the bones meet. A slippery substance called synovial fluid sits between these joints. This fluid helps our fingers move easily. It reduces friction and keeps the finger movements smooth.
We change the pressure inside the joints when we pull or bend our fingers hard. Gases like carbon dioxide, oxygen, and nitrogen dissolve in the synovial fluid. They react to this sudden pressure drop. They form a small gas-filled gap inside the joint. Science calls this process cavitation. This mechanical change creates that familiar popping sound.
Basically, the pressure drops when we pull our fingers. Gas bubbles form in the fluid. These bubbles burst and make that loud pop.