What makes the pop when you crack your knuckles?

The pop comes from gas in the fluid inside your joint. Your knuckles are bathed in a slippery liquid called synovial fluid, which, like all bodily fluids, contains dissolved gases. When you pull or bend a finger joint, the joint surfaces are forced apart and the pressure inside drops suddenly. The gases come out of solution and form a bubble, or cavity, and that event makes the crack. It's a bit like opening a bottle of fizzy water, only very fast.

That's also why you can't crack the same knuckle twice in a row. After a crack, there's a refractory period of about 20 minutes while the gas slowly dissolves back into the fluid.

Scientists have argued for decades about the exact moment of the sound. The old view was that it comes from a bubble collapsing. In 2015, researchers filmed knuckles cracking inside an MRI scanner and saw the sound happen as the cavity formed, with the cavity still visible afterwards. A 2018 computer simulation argued for collapse again, but it hasn't convinced most scientists.

And arthritis? The old warning has no scientific evidence behind it. One doctor, Donald Unger, tested it on himself, cracking the knuckles of only his left hand every day for more than fifty years. Neither hand developed arthritis, and he won an Ig Nobel Prize for it.