[WeirdScience] That satisfying pop when you crack your knuckles isn't your bones. Here's what it actually is

[Beth]

Every joint is surrounded by synovial fluid containing dissolved gases including nitrogen, oxygen and carbon dioxide, and when you pull a joint apart quickly, you create negative pressure that forms a gas filled cavity inside the fluid. For decades scientists debated whether the sound came from that cavity forming or from it collapsing again afterward, but a 2018 study from Stanford and the Ecole Polytechnique in France, using a mathematical model of the joint, found the sound most likely comes from the partial collapse of that bubble rather than its formation. The gas bubble that forms actually increases the joint's volume by 15 to 20 percent and is made up mostly of carbon dioxide, which is also why you can't crack the same knuckle again immediately, it takes time for that gas to fully redissolve back into the fluid.

As for whether the habit causes arthritis, multiple long term studies, including a notable 2011 analysis of crack years, have found no meaningful link between habitual knuckle cracking and joint disease, though repeated forceful cracking may still reduce grip strength or cause soft tissue irritation over time.

What makes the sound when we crack our knuckles?www.scientificamerican.com

[Kayla73]

The 2018 Stanford study finally settling the formation versus collapse debate is satisfying, that's been an open question since basically the 1970s