The joints in your fingers are the most common types of joints in your body. Called diarthrodial joints, they're found in your toes, elbows, wrist, hips, and shoulders. The two bones that come together are protected by cartilage and are surrounded by a thick liquid called synovial fluid, which helps the joints move more easily. Over time, gases like carbon dioxide dissolve into the fluid, creating a bubble and pushing the bones farther apart; cracking joints was long thought to be a release of those gases, effectively popping the bubble. Though this has been the prevailing theory, scientists didn't all agree, and past attempts to capture images of cracking joints have been low-resolution or not quick enough, as the cracking itself takes less than 310 milliseconds.