When you were young, you probably first learned of double-jointedness when a sibling or a classmate showed you the insane way 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 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.
Though the trait is uncommon, double-jointedness surprisingly does not fall into the category of recessive genes, meaning that people with double-jointed parents are more likely to inherit the trait. According to research, side effects of hypermobility syndrome include increased risk for anxiety, IBS, and asthma, so being a human ragdoll isn't always a blessing. Thankfully, many double-jointed people (and at least one dog) have learned to make the most of their circus talents.