Have you ever wondered what happens when people crack their knuckles?
Well scientists have used medical scans to try to capture each "pop" or "crack" in action.
They found the noise was linked to gas collecting in the joint spaces.
As people flex or pull their fingers, the joint spaces get bigger and gas rushes to fill the gap.
This makes a loud "crack'' sound.
The experiment was done using MRI scanning technology.
On the scans, there was no sign the gas bubbles had popped or disappeared.
The bubbles stayed there even after the cracking noise had occurred but were hidden from view when the finger went back to its normal position.
Now, researchers want to use even more advanced MRI scanning technology to understand what happens in the joint after the pop, and what it all means for health.