Now that we are several paragraphs deep in the nuances of bubble collapse, are you thinking what I’m thinking? I have seen many bubbles form and pop in life, and I have never heard anything as loud or sharp as the sound of a knuckle cracking. Really? Bubbles?

It turns out that the bubbles forming in knuckles are not quite like the bubbles we ordinarily encounter in water or air. After all, there is no air rushing your knuckle to form a bubble. Rather, these bubble form in a process called cavitation, when negative pressure “pulls” the joint fluid apart.

“It’s like the rupture of water,” says Philippe Marmottant, a physicist at the Laboratoire Interdisciplinaire de Physique, in France. Marmottant has studied how cavitation in trees will produce ultrasonic cracks. Cavitation can also happen when a boat’s propellers tear through the water, and its collapse can generate shock waves strong enough to tear a hole through metal propellers over time.

When cavitation occurs in the knuckle, the negative pressure causes gases dissolved in the synovial fluid to come out. It takes time for the gases to dissolve back in, which is why you cannot crack your knuckles again right away.

Suja did not mathematically model how bubbles first form in synovial fluid. He acknowledges this is the next step for ruling out bubble formation as the source of audible cracks.

Robert Boutin, a radiologist at the UC Davis, who has studied knuckle cracking with ultrasound imaging, says he still thinks bubble formation causes cracking based on his research, which came on the tail of the 2015 MRI study. “It’s very interesting, but incremental,” he says of Suja’s mathematical model, given the small number of participants involved. “Stay tuned in this space.”

The scientist who led the MRI study, Greg Kawchuk, called the new model “exciting.” He acknowledges that MRI is limited by how infrequently it can take an image—only once every 310 milliseconds in his case. In his study, the knuckle crack itself was captured in just one frame of the MRI video. It’s possible the bubble may be forming and partially collapsing all in that time.

“I don’t think we completely understand it yet,” Kawchuk says. “Anything for the topic area is a win and adds to what we need to know about this really important phenomenon.”

What’s so important about knuckle cracking? “It’s entirely speculative on my part,” Kawchuk replied, then proceeded to tell me that perhaps knuckle cracking requires a checklist of factors like joint alignment or a certain amount of synovial fluid. Not everyone can crack their knuckles and not every knuckle will crack. “Maybe if you can actually do this, it’s a sign of everything in the joint in there is working properly. Maybe what we’ve thought all along, joint cracking is bad for me—it might actually be the other way around.”

I wish I could have felt vindicated in the moment. But I have never been able to crack my knuckles.