I've always wondered about this

Why do knuckles crack? There's more to it, but here's the core reason:

A protective fluid cushions most of the joints in our bodies.

Inside a capsule that safeguards bones connected at a joint, synovial fluid keeps the cartilage, tissues, and muscles lubricated and well nourished. Nutrients float inside the fluid, along with gases, such as oxygen, nitrogen and carbon dioxide.

As you bend your fingers, the joint capsule stretches. To make more room for the stretch, gases release out of the fluid. The pop of your knuckles is the sound of gas as it bubbles out of the fluid, scientists say. Before your knuckle can crack again, the fluid must reabsorb the gas.