Synovial fluid

Synovial fluid is the thick, egg-white-like liquid held inside the capsule of a freely moving joint. It lubricates the surfaces, absorbs shock, and carries nutrients to cartilage that has no blood supply of its own.

It matters because its behaviour changes with movement: it becomes less viscous and circulates more as the joint moves, which is the physiological basis for a warm-up genuinely making a joint feel better rather than merely feeling like it does.

In practice this is also why prolonged stillness makes joints feel stiff and why gentle movement relieves that stiffness faster than rest does. The popping sound of a cracked knuckle is gas coming out of solution in this fluid — a harmless phenomenon, not a sign of damage.