Step 1 : Introduction to the question "What makes the sound heard when "cracking" one's knuckles?"
Joints are points where two bones contact each other on opposite sides of a cartilage surface. A joint capsule encases the cartilage. The lubricant inside the joint capsule, called synovial fluid, contains dissolved gases, such as oxygen, carbon dioxide and nitrogen. The amount of synovial fluid limits the joint motion. When pressure inside the joint capsule drops, the dissolved gases come out of the solution. This increases the volume of the synovial fluid, and the mobility of the joint. The noise comes from the rapid expansion of gases that allows the joint capsule to stretch a bit further. Source: ScientificAmerican.com
