Why do my knuckles sometimes make a cracking sound if I bend them?

Knuckles—as well as your knees, back, and neck—sometimes make a popping or grinding sound thanks to the body’s synovial fluid, which acts as a lubricant in your bone joints. This fluid contains the gases oxygen, nitrogen, and carbon dioxide. When you pop or crack a joint, the joint capsule is stretched, gas is rapidly released, and bubbles form. If you want to crack the same knuckle again, you have to wait until the gases return to the synovial fluid. Other sounds, like snapping or the plucking of a rubber band, sometimes happen when your ligaments temporarily slide off your bone as you move your joints.