It's one of the most addictive things humans can do. Everyone does it. And it feels so good when you get to do it. We're talking about cracking your joints. Researchers have alway been curious as to where the sound of joints cracking comes from and put a hand under MRI to find out (and for us to see visualized in real time). They've figured it out.
Cracking sounds emitted from human synovial joints have been attributed historically to the sudden collapse of a cavitation bubble formed as articular surfaces are separated. Unfortunately, bubble collapse as the source of joint cracking is inconsistent with many physical phenomena that define the joint cracking phenomenon. Here we present direct evidence from real-time magnetic resonance imaging that the mechanism of joint cracking is related to cavity formation rather than bubble collapse.
You can read more here.