Q. What cause one’s knuckle to crack?
A. Your finger bones are separated by small pads of cartilage, and in between these pads and the bones are small pockets of thick liquid. When you bend your fingers, the bones pull away slightly from the pads of cartilage, forming a vacuum between the bone, the pad, and the surfaces of the liquid. AS the bending continues and the bones are pulled farther apart, the surface tension of the liquid eventually gives way and the vacuum bubble bursts, making the cracking sound you hear.
