Can you bend your thumb backwards until it touches your wrist? If you join your hands behind your back, can you lift them over your head without letting go? If you can, you might be what some people call "double-jointed." But, A Moment of Science wonders, how could anyone have double joints?
Double-jointed people don't really have two joints. Any joint, like the thumb joint or the shoulder, can be called "double-jointed" if the tendons and ligaments surrounding the joint allow a wider range of motion than is usual.
Joints are created whenever two adjacent bones meet. They have fibrous bands of connective tissue called ligaments that stabilize the joint. Connective tissues also form the tendons that join the bones to the muscles. Ligaments and tendons usually restrict the movement of the joint, but in some people, these normally restrictive tissues can be quite flexible and allow the extra movement, which we call "double-jointed." Practice can increase a joint's natural flexibility just the way stretching can increase your muscles' flexibility.
While flexible joints are very useful to contortionists, some joints can become so loose that they are unstable. This allows the bones of the joint to slip out of their sockets, which can be very damaging. Strengthening the muscles around the joint can stabilize it and still allow you to show off your trick joint at parties.