Why do knuckles crack?

Knuckles crack because the bones in your fingers are separated by small pads of cartilage, and in between these pads and the bones are small pockets of a thick liquid. When the fingers are bent the bones pull away slightly from the pads of cartilage and a vacuum forms between the bone, the pad, and the liquid's surface.

As you continue bending your knuckles and the bones are pulled farther apart, the liquid's surface tension eventually gives way and this vacuum bubble bursts - making the cracking sound you hear.

This process is similar to pulling a rubber suction cup off of a smooth surface.