How do your joints "POP" when you crack your knuckles?

Answer 1

When you pull on your finger, to 'crack' the joint, you make the joint space bigger. As a result, the pressure inside the joint space drops, the ligaments get sucked inwards, as the pressure drops, a bubble (mostly carbon dioxide) appears, in just a few thousandths of a second. As this gas bubble pops into existence, it will make a popping sound, which is the first of two sounds.
Because the joint space suddenly has a bubble in it, the liquid just as suddenly pushes on the ligaments - snapping them back outwards to their original position. This gives the second sound.

Answer 2

All of the joints in our bodies are surrounded by synovial fluid, a thick, clear liquid. When you stretch or bend your finger to pop the knuckle, you are causing the bones of the joint to pull apart. As they do, the connective tissue capsule that surrounds the joint is stretched. By stretching this capsule, you increase its volume. And as we know from chemistry class, with an increase in volume comes a decrease in pressure. So as the pressure of the synovial fluid drops, gases dissolved in the fluid become less soluble, forming bubbles through a process called cavitation. When the joint is stretched far enough, the pressure in the capsule drops so low that these bubbles burst, producing the pop that we associate with knuckle cracking.

Answer 3

it's air releasing from the joints.

Answer 4

It's the grinding of your cartilage.