What's that sound? Depending on whom you ask, the crackle of popping joints is either the sound of sweet relief or the noxious tones  of a stomach-turning habit. Really, though. What's that sound? I mean, why does bending your joints in a certain way make them pop like that? Scientists have offered  several explanations, including rapidly stretching ligaments, and in severe cases, actual bones  grinding against each other. But the most common explanation for why your stretched-out joints sound like bubbles popping is that, well, there are bubbles in there. The joints in your fingers  are the easiest ones to crack, but many people also crack the joints between vertebrae in their neck and back, and even their hips, wrists,  shoulders and so on. All these joints are synovial joints, and they're the most flexible  ones in your body. The space between the two bones is filled with a viscous liquid, synovial fluid, which contains long, lubricating molecules, like hyaluronic acid and lubricin. Synovial fluid is more or less the texture of egg yolk and its primary purpose  is to cushion the bones and help them glide past each other. It also contains phagocytic cells that help clean up any bone or cartilage debris that ends up in the joint. But the reason it's important  for knuckle cracking is that, like other fluids in your body, it contains lots of dissolved  gas molecules. Knuckle-crackers know that  to get that satisfying pop, you stretch the joint farther than it normally goes by bending your fingers  backwards, for example. When you do that,  the bones move away from each other. The space between bones gets bigger, but the amount of synovial fluid  stays constant. That creates a low-pressure zone that pulls dissolved gases  out of the synovial fluid, just like the carbon dioxide  that fizzes out of soda when you twist open the cap. Inside the joint, the escaping gases  form a bubble with a pop. But the bubble doesn't last long. The surrounding fluid presses on it until it finally collapses. The bubble's gases scatter  throughout the synovial cavity and slowly dissolve back into the fluid over the course of about twenty minutes, which is why it can take a while before you can pop the same joint again. Some scientists think  there may actually be two pops. One when the bubble forms,  and another when it bursts. Popping a joint temporarily enlarges it, which may be why dedicated  knuckle-, neck- and back-crackers say the habit makes their joints feel looser and more flexible. But you may have heard from  a concerned relative or annoyed officemate that cracking your joints  will give you arthritis. A doctor named  Donald Unger heard this, too. So, determined to disprove  his mother's warnings, he cracked the knuckles  of his left hand repeatedly for 50 years, while the right-hand  knuckles went unpopped. 36,500 cracks later,  both hands were arthritis-free. For this selfless act  of devotion to science, Dr. Unger received an Ig Nobel Prize, a parody of the Nobel Prize that recognizes wacky, but weirdly fascinating, scientific accomplishments. Unger wrote that his results should prompt investigation into other parental beliefs, like the importance of eating spinach. The jury's still out on that one. As for knuckle-cracking, one study suggests  that all that joint stretching and bubble bursting can cause your hands to swell and weaken your grip. But the biggest proven danger seems to be annoying those around you.