The cracking sound you hear when you pull on your fingers is caused by the release of negative pressure within the joint.
Inside every joint in the body, a gooey substance called synovial fluid coats the surfaces of cartilage to protect them. Just as two discs coated with mineral oil stick together and can only be pulled apart with a bit of force, so it is inside the joint. When you pull, you break the vacuum seal created by the synovial fluid and the breaking of that seal causes the popping sound -- from the bursting of bubbles of gases dissolved in the fluid.
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