We all have that one friend who’s double jointed -- they can bend their fingers or whatever backward at what looks like an excruciating angle.

Makes for a neat, if kinda gross, party trick! But what does “double-jointed” actually mean? And why do some people have this ability but not others?

No one literally has double the number of joints, but some people do have joints that are more flexible than the norm. You can increase your flexibility through training and practice, of course, but some people are just born this way, and the technical term for this is hypermobility.

One way this can happen is if a person’s joints are a bit shallower than usual. A joint, as you probably already know, is where two bones come together. The most flexible kind of joint in our bodies is the ball-and-socket joint -- that’s what’s in our elbows, hips, and shoulders.

In a ball-and-socket joint, a ball-shaped projection at the end of one bone fits into an indentation at the end of another. And the shallower that indentation is, the greater the range of movement the joint has. So if your friend at the party has particularly bendy elbows, that could be because the socket in that joint is just unusually shallow.

But hypermobility can also be the result of especially elastic cartilage, the firm but flexible tissue that can be found in most joints. Or it could be a function of unusually flexible ligaments, the connective tissue that holds bones together. A few genetic conditions -- such as Ehlers-Danlos Syndrome -- can cause defects in these tissues that lead to joints being unusually loose.

And hypermobility does have its downsides. For one thing, it can make it easier than usual to accidentally dislocate joints. This is when the ends of the bones pop out of their normal positions, which is usually really painful and requires a doctor to fix.

Ehlers-Danlos Syndrome in particular comes with a host of unpleasant symptoms besides the flexibility boost, like skin that bruises easily, wounds that are slow to heal, and early-onset arthritis. So even though hypermobility may look cool, it’s more than just a party trick.

Thanks to Patreon patron Chris A for asking, and thanks to all of our patrons who keep these answers coming! If you’d like to submit questions to be answered, you can go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!