Many of your bones meet at joints. These allow you to bend and move. At a joint, the bones are held in place by strong straps, called ligaments. The ends of the bones are covered in pads of tough gristly cartilage, to prevent wear and tear. They are kept "oiled" by a special fluid, called synovial fluid. If you are "double-jointed", it just means that you have extra long ligaments in your joints and can bend them farther than usual.