Water on the knee is a not a condition in itself, but an everyday term which covers a range of problems that can affect the knee. Today, doctors refer to water on the knee as a knee effusion or bursitis.
A knee effusion is an inflammation of the flesh around the knee joint, sometimes caused by a build up of fluid, although it is unlikely that this fluid would leak from your legs. One knee, and perhaps the rest of the leg, will appear larger than the other and the knee will often be stiff, and you will be unable to straighten the leg. The knee will be painful depending on how the effusion came to happen (such as a recent injury to the knee, as opposed to arthritis or repetitive straining) and the same applies to possible bruising. In some cases, the swelling may be caused by some form of disease.
How to Treat a Knee Effusion
Most cases of bursitis can be treated at home. Resting the affected area, using an ice pack (a frozen bag of vegetables wrapped in a tea towel works well) to reduce inflammation, and taking painkillers such as paracetamol and ibuprofen can help relieve your symptoms. If the swelling continues, and the afflicted knee reddens and becomes warm to the touch, then you will need to make an appointment with your physician. Here's a film showing some exercises you can do to strengthen the knee joints: