Tickling has such a history that Aristotle wrote about it in 350 BC: "The fact that human beings only are susceptible to tickling is due 1) to the fineness of their skin and 2) to their being the only creatures that laugh."

He didn’t have it quite right, and centuries later, there's still a lot of mystery around tickles. But many scientists have actually delved into the topic. Here's what we know.

What is a tickle?

The definition of a tickle is a little loose, because it's both a noun and a verb. It’s a sensation you get either when something mildly moves across your skin or when someone attacks you in a vulnerable place for fun, says Glenn Weisfeld, a psychologist who has studied human emotions and tickling.

There are actually two types of tickles, known as knismesis and gargalesis. Knismesis is a light sensation, like what you'd feel when a feather is run across your skin. You want to brush away the sensation because it's an irritating feeling. The other type of tickling, gargalesis, is what happens when someone is tickled more aggressively, like by another person.

There are actually two types of tickles, known as knismesis and gargalesis

The differentiation dates back to 1897, when psychologist G. Stanley Hall and his coauthor Arthur Allin defined the two types for research purposes. The biggest distinction between them: gargalesis is the kind of tickle you can't do to yourself, but you can certainly give yourself knismesis. Separating the two in research helps scientists clarify the type of tickles they're studying.

Why are we ticklish?

Scientists have various ideas about why, but aren't completely sure. One is that tickling is rewarding: it causes laughter, and, simply put, people like laughing.

Tickling is also a form of social bonding. It's one of the early forms of communication between parents and children and is a way young children play around with their friends. So perhaps, researchers say, tickling is a way to form connections with people. (But this reason doesn't apply to everyone, as some people find tickling painful.)

Another idea is that we've evolved to be ticklish as a way to protect vulnerable spots from attack. For example, because your stomach is ticklish, you're more aware that you need to protect it if you're facing some sort of threat.

Why can't you tickle yourself?

You can’t tickle yourself because you know it’s coming. Lack of surprise somehow seems to trip up the tickling response, because your brain knows ahead of time that you're going to cause a sensation to your own body.

one group of people who can actually tickle themselves: people with schizophrenia

Evidence from brain scans supports that tickling oneself provokes a different reaction. The two main parts of the brain involved with tickles, the somatosensory cortex (which processes touch) and the anterior cingulate cortex (which processes happy things), are much more stimulated when people are tickled by others than by themselves.

But there's one group of people who can actually tickle themselves: people with schizophrenia. Though it's not totally understood why this happens, research has shown that, on average, their brains can't differentiate between self-generated and externally generated touches, which might be what makes them extra sensitive to their own touch.

Why does tickling make you laugh?

It seems weird that if we're trying to defend ourselves against a (friendly) attack, our natural reaction is to laugh uncontrollably. There are a handful of ideas about why this happens, but no definitive answer.

Some evolutionary researchers say that laughing when you're being tickled is a defense mechanism. By looking at MRI scans of people being tickled, scientists have determined that the hypothalamus — which is responsible for fight or flight responses — comes into play when you're being tickled. Some think that laughing while under a friendly attack could be your body's way of signaling your submission to the person touching you in an effort to stave off further tickles.

Another idea is that laughing is a response learned during childhood. If young children are being tickled in a playful setting in which they're already laughing, they might come over time to associate tickling with laughter.

How do researchers test tickles?

There's a surprising amount of research on tickles. Here's some of the most amusing.

1) Back in 1872, Darwin actually wrote about tickling, comparing how people react to tickles with how people react to jokes or humor. He suggested that in order to be tickled, you must be in a good mood, be surprised, and only be touched lightly.

2) One researcher in the 1940s studied tickling using his two kids as test subjects. Ohio scientist Clarence Leuba of Antioch College tickled his two infant children while wearing a mask so they couldn't tell if he was laughing. His kids still laughed, though, and Leuba concluded that laughter is likely a natural response to tickling, not a learned one.

3) Decades later, in a 1999 study, University of California–San Diego researchers blindfolded 32 undergraduate students and gave them a heads up that a robotic hand would tickle their feet once, then a person. Then, secretly, the person tickled them both times.

The students reacted the same way when they thought a machine was tickling them as when they thought it was a real person. The researchers concluded that the response to being tickled is a natural reflex rather than a social one that comes from an interaction between two people. (This is still up for debate, like the other ideas about why we're ticklish).

Why are some body parts more ticklish than others?

The places you’re most ticklish tend to also be places most vulnerable to a physical attack — ones without bones to protect them, like your stomach. This makes sense according to those who believe that tickling has something to do with learning to defend oneself.

And researchers have actually tried to catalog them over the years. In one study, from 1997, University of California–San Diego researchers had an assistant tickle 72 college students and found that they were — not surprisingly — most ticklish on their underarms, waists, ribs, and the bottoms of their feet. Another study, this one from 1897, involved tickling 700 children. The research found that the kids were most ticklish on the bottoms of their feet, and on their underarms, neck, and chin.

Of course, there’s always some variation from individual to individual. Some people aren't very ticklish at all. Whether tickling makes you giggle or cringe, know that you might laugh yourself silly before science figures out all the mysteries about the funniest way we defend ourselves.