Where two bones must slide freely, a capsule seals the joint and a soft lining makes synovial fluid. The amount is small. It is a film between the cartilage surfaces, not a pool you could pour out.
That film cuts friction to a level engineers envy, and it also feeds the cartilage that has no blood vessels of its own. Less movement means less stirring of the fluid.
A stiff morning joint is often this system waking up. A few bends spread the film again, and the hinge remembers how to glide.
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