IT'S something we have wondered for years - why do our knuckles crack?
We've all heard the old wives' tales that knuckle cracking causes arthritis, which puts most of us off doing it, but the route cause of those popping sounds has never been found.
Yet the phenomenon has likely been around for all of human history and scientists have been looking into it since the 1900s.
But now a new paper, published in the journal Scientific Reports, claims to have found the cause of the noise - the collapse of tiny air bubbles in the fluid that surrounds the joints.
The bubble is known as a cavitation and forms in the synovial fluid that lubricates the joints, according to Live Science.
When the fluid gets pulled apart, like when your knuckles move, it creates a decrease in pressure and gas-filled spaces occur.
And when the bubbles pop it creates a cracking noise.
But another study, in 2015, had an opposing view that the popping noises occurred when the air bubbles formed, not popped.
Experts from the University of Alberta in Canada reported they had found the cause of knuckle cracking using an MRI machine to watch the process happen.
They found that cracking sounds coincided with the formation of air bubbles.
But Vineeth Chandran Suja, a doctoral candidate in chemical engineering at Stanford University who studies bubble dynamics, was not convinced.
The sound of the cracking was too large to be coming from a bubble formation, he wrote in the new paper.
Instead he and his co-author, Abdul Barakat, developed a mathematical way to tell exactly what happens when a joint pops.
They found that the noise occurs when the pressure on the synovial fluid increases and the air bubbles shrink from large to small.
"Our model shows that the acoustic signature from a partially collapsing bubble inside a cracked joint is similar to ones we observe experimentally," Suja told Live Science.
"Our model is the first mathematical model and as such there are a number of future directions that can be pursued to develop improved models to confirm beyond doubt the source of the cracking sounds."
But the bubbles didn't disappear completely, they simply shrunk in size.
All it takes is the bubbles compressing for a cracking sound to come from our knuckles.