So some people find immense pleasure in cracking their knuckles and every other joint in their body...while others cringe at the thought of that sound. Scientists have wondered for a while what actually causes the sound, and they finally think they've cracked the code.
The space in between the knuckles is filled with something called synovial fluid. When you move your fingers and create space in-between the joints, bubbles produce within the fluid and create the popping sound.
The key word is here is that scientists THINK they've figured it. This is only a mathematical model that hasn't been proven through tests.