Cracking your knuckle makes a vacuum inside your joint, and the pop is the fluid tearing open. Each knuckle sits in a sealed capsule of synovial fluid, a lubricant with gas dissolved in it like a closed can of soda. Pull the bones apart and the pressure inside drops. At a critical point the fluid can't stretch any further and a gas cavity rips open in under 310 milliseconds. That's the sound. Then you wait 20 minutes before it will crack again. That's roughly how long the cavity takes to…