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You know what's like super funny? Laughter. Laughter is a physiological response that involves at least 15 facial muscles, the respiratory system, the brain's limbic system, and--if the joke is really, really good--even your tear ducts.

But laughter doesn't always indicate happy times; it can be a sign that something is seriously wrong with your brain. Gelastic seizures, or uncontrollable laughter or crying, can indicate the presence of brain tumors or other conditions like pseudobulbar affect, a neurological condition that can effect stroke and brain trauma survivors and MS patients. These conditions are sometimes called emotional incontinence because the sufferer can't control the outbursts, which often have no root in have they're felling. 

And how we feel, especially in groups, seems to be what laughing is all about. Some laugh researchers--and, yes, they are things known as gelotologists--think that much of our laughter is rooted in strengthening social bonds. We're much more likely to laugh in a group than we are alone, and we tend to laugh more easily around friends and family. That shared experience brings us closer, makes us feel part of the group. We also laugh to express relief or to ease our nerves in stressful moments. 

Researchers theorize that there are a few specific reasons for laughing. First, there's the incongruity theory which maintains that it's the element of surprise that triggers laughter, whether it's an unexpected punchline or your friend tripping over a throw rug. So like say you're watching people walk through a room all day; your brain registers this a predictable and boring behavior. Then, your friend walks in, trips on a rug, and drops a box of ping-pong balls. When you hear that your friend hasn't say, fallen on a bag of rusty knifes, you'll find the fall hilarious because it was sudden and unexpected and incongruous to the sting of people safely walking by, and because ping-pong balls. Babies and little kids go for this kind of laughter a lot; they think think really simple, unexpected things my ah, my face is here, it's there for five consecutive hours it's still funny. You're like using a banana as a telephone.

Now, if you're the one who tripped on the throw rug, you're much more likely to laugh in surprise if you see your friend laughing with you. This goes back to that shared laughter as a social bond thing. You're probably feeling pretty embarrassed and tense, but you're also relieved that you aren't hurt. That's where the relief theory comes in.

Laughing is like a mental mini break. Your brain is constantly working, taking in all sorts of information and ordering around the body; sometimes it just needs a happy surprise. This is particularly handy is stressful moments. The whole Hollywood wise-crack in the middle of a suspenseful scene phenomenon is predicated on this idea. When Han Solo is in mortal danger, he cracks a joke to lighten the mood; he needs it, Chewie needs it, the audience at the edge of their seats need it.

Humor helps us cope with stressful situations; it sort of recharges our brains to face the task at hand. Scientists call this releasing cognitive energy; the rest of us call it comic relief. 

But now back to you tripping on that throw rug. If everyone in the room is laughing at you and none of them are your friends, they may be proving the superiority theory of laughter. It means that they're laughing at your misfortune and probably means that they're kind of a bunch of jerk faces. Superiority laughter still promotes bonding, in an us vs. them sort of way. It doesn't show much goodwill though. 

Teenagers make fun of their parents, and, you know, each other, and pretty much everyone, and so do lots of adults as well. But our teen years are usually awkward and confusing, and superiority laughter might help ease some of that pain.

So, simply put, we laugh hardest at what we know best and what stresses us out the most. And maybe that's why it's great for you both physically and emotionally. It reduces the release of stress hormones that jack those flight-or-flight feelings, it lowers blood pressure and oxygenates your blood flow, it even increases your T-cell levels that help your immune response and B-cells that produce antibodies. Also laughing a hundred times is estimated to burn as many calories as a 15-minute bike ride. So, you see: laughter is not the best medicine, but it's not a bad medicine either. 

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