The knee joint is the mobile connection between the thigh bone (femur) and the shin bone (tibia). Cartilage lines the knee joint, and strong ligaments and tendons hold the moving surfaces together.
The knee works roughly as a hinge joint, although actual knee movement and geometry are far more complex than a simple hinge. Knee motion involves rotation, sliding and many other types of movement.
The joint lining (called synovium) secretes a grease-like liquid called synovial fluid, which reduces friction in the knee joint. This fluid is produced in other joints also, and it is the equivalent of biological “motor oil.” This oil-like fluid keeps the knee joint lubricated.