When you were young, you probably first learned of double-jointedness when a sibling or a classmate showed you the bizarre ways they could twist their fingers. As you grew older, you realized double-jointed people can achieve all sorts of theoretically impossible body contortions, and that shocking double-jointed photos depict more than just bendy hands and feet.
People with hypermobility - the condition that makes double-jointedness possible - don't actually possess double joints; hypermobility syndrome works by allowing a person to extend a bone to its fullest extent without feeling any discomfort. The trait is fairly common and appears to be inherited. The extra collagen present in a double-jointed person's limbs grants them great flexibility, not to mention a bend and snap that's better than Elle Woods's best.
Yet, being a human ragdoll isn't always a blessing. According to research, side effects of hypermobility syndrome include increased risk for anxiety, IBS, and asthma, Thankfully, many double-jointed people (and at least one dog) have learned to make the most of their inherent talents.