Why are some people double-jointed?

Q Why are some people double-jointed?

Matthew S., New Jersey

This is kind of a trick question. No matter how much someone can pull back his elbow or twist her finger, there's no such thing as being "double-jointed." People who seem that way have the same number of joints as everyone else; it's just that their joints are more flexible.

But some "double-jointed" people actually have Ehlers-Danlos syndrome (EDS), a condition in which the connecting tissue and binding muscles are very elastic, like rubber bands. A hereditary problem, EDS can lead to pain, dislocations, bruises, and scars. It can also affect the organs.

Some people are so flexible, they seem "quadruple-jointed." For example, a contortionist can bend like a pretzel and performs tricks for audiences. Contortionists build on their natural flexibility and practice movements to become good at what they do. The performers say anyone can be more flexible with practice.