So what causes the “popping” sound?
Your knuckles are joints covered by a capsule (the joint capsule). Within this capsule there is fluid, which acts as a lubricant and also contains nutrients for the adjacent bone surfaces. A variety of gases are continuously dissolved in this fluid. When one cracks a knuckle, the stretching of the capsule lowers the pressure inside the joint and creates a vacuum, which is filled by the gas previously dissolved in the fluid. This creates a “bubble” which then bursts producing the “popping” or “cracking” sound. It takes a while until these gases are re-dissolved in the fluid, which explains why knuckles cannot be “re-cracked” immediately.