Asked by: Steve Shaw, Durham
A 2015 study at the University of Alberta, Canada took real-time MRI images of finger joints as they cracked. The researchers found that as the surfaces of the cartilage pull away from each other, they create a site for a tiny bubble of dissolved gas (mostly CO2) to form in the joint’s lubricating fluid. The sudden expansion of the bubble creates the popping sound.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.