When you crack your knuckles what cracks is there air bubbles?

The cracking or popping sound you hear when you crack your knuckles is not the sound of air bubbles bursting. Instead, it is the sound of synovial fluid, a thick, viscous fluid that lubricates your joints, filling a gas bubble in the fluid that surrounds the joint. This gas bubble is nitrogen oxygen, and carbon dioxide. When the joint is cracked, the pressure inside it is rapidly decreased, causing the gas to come out of solution and form bubbles. The bubbles then burst, creating a popping sound.

Cracking your knuckles is not generally harmful, but it can cause damage to the joints over time. If you are concerned about cracking your knuckles, you should talk to your doctor.