Some people like the sound of knuckle-cracking and others loathe it, but for years there has been disagreement among scientists about what actually causes it. Researchers said they have settled the issue of what occurs inside knuckles to trigger the popping sound, thanks to magnetic resonance imaging experiments that they dubbed the "pull my finger study".It turns out the cracking is caused by the rapid formation of a gas-filled cavity within a slippery substance called synovial fluid that lubricates the space between the finger bones, they said. The knuckle-cracking happens in less than 310 milliseconds. ..