The knee has 11 bursae, and 2 of them are especially susceptible to bursitis. When a bursa becomes inflamed it is called bursitis. An inflamed knee bursa can fill with excess fluid, causing swelling, or water on the knee. The swollen knee may feel “squishy,” like a water balloon. It may or may not be tender and painful.
https://www.arthritis-health.com/types/general/what-causes-swollen-knee-water-knee

