The ability to laugh is a lovely human characteristic. But it’s quirky. Some people laugh when they’re sad. For others, laughing is exercise—literally. Or a sign of disease! What do you really know about laughter? Take this quiz to find out!

When you're with people rather than by yourself, the chance that you'll laugh...

We're 30 times more likely to laugh when we’re with friends than when we’re alone. Like dominoes, laughter can be a chain reaction—one person’s laughter can start a whole group giggling. When something catches everyone’s attention, say, a hilarious video, each person’s brain processes a strong emotional reaction in a similar way. Everyone’s in sync—it’s how social bonds are formed.

Even if you don't get the joke, you'll still smile.

Just hearing other people laugh—even if you don't know why they are laughing—triggers the brain to tell our mouths to smile. Scientists have found that people smile when they hear a positive sound such as laughter, but they don’t form any special facial expression in response to negative sounds like screaming. Indeed, that "mirroring" of positive sounds helps explain how smiling and laughing spread from person to person.

Laughter is so healthy that it's now included in some...

Laughter yoga began in India in 1995, and it's now spread to the US. Laughter, clapping and childlike play are incorporated into yoga breathing exercises. Proponents claim that it improves circulation and mood, stimulates the immune system, reduces pain and relaxes muscles. One study concluded that a six-week laughing yoga program helped improve mood and lower blood pressure among older adults.

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Uncontrollable laughing can be a sign of illness.

People with a condition called pseudobulbar affect (PBA) sometimes laugh (or cry) uncontrollably—even though nothing particularly funny or sad has happened. PBA is a neurological condition that results from having a brain disorder such as Parkinson’s disease, dementia or traumatic brain injury. Having PBA episodes can be frustrating, confusing and embarrassing. Medication and behavioral techniques can help control the symptoms.

Relaxed, natural laughter increases pain tolerance, studies show. It does so by...

The physical exertion involved in laughing releases endorphins, the body’s own mood boosters, much the same as running or other forms of exercise. So laughter actually produces a similar sensation as so-called runner’s high.

Watching funny videos...

Funny videos are more than mere fun. Watching one can improve short-term memory, find California researchers. The video watchers also had less of the stress hormone cortisol in their saliva. Why that's important: Stress interferes with the brain’s hippocampus, where new memories are formed. Upshot: You can feel good about watching funny online cat videos!