The Real Reason You're Addicted to Cracking Your Knuckles | The Thirty | SAN ANTONIO XYZ | Scoop.it
“Many believe that cracking knuckles is the bones realigning, which couldn’t be further from the truth,” says Weiss. Actually, the process has to do with your joints. 

“Knuckling cracking is the audible sound that occurs when the joints of your fingers are stretched,” says Curda. “These joints are called synovial joints and are surrounded by fluid-filled capsule.” That fluid, called synovial fluid, is there for lubrication, and is made up of dissolved gases—mostly nitrogen, Curda says. 

When you pull or bend your fingers (aka the two most common “cracking motions”), you stretch that fluid-filled capsule. This decreases pressure inside the gas-packed fluid, causing a small nitrogen bubble to form and then instantly pop. “The collapsing bubble is what causes the audible crack that we hear,” says Curda.