Everybody had that one kid in their third-grade class who could do gross party tricks, folding her body into a pretzel or twisting his fingers into strange configurations. Double-jointedness is certainly weird. But is it dangerous? That really depends on the person, and what else their body is doing.

The medical term for this extreme bendiness is joint hypermobility (JH). Experts have recently suggested sorting this trait into three different types. There’s the harmless kind (called asymptomatic JH); the kind that can cause pain (hypermobility spectrum disorders, or HSD); and the kind that’s actually a symptom of some other underlying medical condition.

Asymptomatic JH is very common. Many party-trick kids, super-flexible ballet dancers, and circus contortionists are not in pain, nor are their joints dangerously loose. But for people with HSD, that hypermobility is a problem, or it can become one as they get older. The same flexibility that can make a person a better dancer or athlete can increase their risk of injury or developing arthritis later down the line.

“When you ask those people in 10, 20, 30 years later how they’re feeling, it's not infrequent that those individuals have joint troubles,” rheumatologist David Bornstein told Real Simple. “Either they stretched their tendons so far that now they’re aching, or they’re experiencing some degeneration in their joints because their cartilage has seen more pressure than it normally would.”

The third type (when JH is a symptom of an underlying condition) is the trickiest to diagnose. JH can be a symptom of disorders like hypermobile Ehlers-Danlos Syndrome (hEDS), a genetic condition that affects the body’s connective tissue. Tendons and ligaments are just two types of connective tissue; it’s also found in our skin, cells, organs, and brain, which means that people with hEDS might have lots of different and seemingly unrelated medical issues at once.

Experts don’t think hEDS is rare, but it is pretty obscure, which means it typically takes doctors a long time to arrive at a diagnosis. People with asymptomatic JH and mild HSD can help protect their hypermobile joints with strength training and conditioning under a doctor or physical therapist’s supervision. It’s important is to find safe exercises that don’t create additional stress on already vulnerable joints. Though it seems counterintuitive, many doctors advise hypermobile patients to avoid stretching and yoga, which can push fragile joints too far.