Okay, think of the last time you were physically tickled.

Whether it was done by a friend or family member, they were probably trying to make you laugh.

But for most of us, especially the most ticklish among us, being tickled isn't necessarily fun, or funny.

So why do we laugh?

And where did this whole bizarre reflex come from?

Tickling actually refers to two different types of sensations.

A light tickling, like when something just barely brushes your skin and makes you want to scratch or rub the area.

This is called Knismesis, or a moving itch.

And it has a helpful role: like if you have an insect on you, your response to that tickling sensation can help get it off quickly and prevent a bite But when we talk about tickling, we're usually talking about the other sensation: where someone touches you with more pressure, causing you to laugh involuntarily.

This is called Gargalesis, and some people are more susceptible to it than others.

While we don't know exactly why, we have some ideas.

Most people are ticklish in areas like the neck, ribs, inner thighs, knees, or feet, but these aren't the most sensitive areas of the body.

If being tickled was based on sensitivity, people should be ticklish on their face, or the palms of their hands.

Instead, our ticklish areas are the ones most vulnerable if we're in a fight or being attacked.

So, tickling may be evolution's way of teaching even very young children to protect their weak points.

This also explains why we can't tickle ourselves.

It's hard to attack yourself when we can predict the outcome of our own actions.

Being able to tickle yourself would serve no evolutionary purpose.

What this doesn't tell us, is why we laugh when we're tickled.

But think about it: imagine tickling a kid, and they start screaming and crying.

You'd probably stop, right?

But laughter is fun!

Scientists think the laughter response is to encourage tickling, and the self-defense training that comes with it.

And it's not just humans who participate in this strange activity.

Other great apes make a laugh-like noise when tickled, which is thought to be the evolutionary precursor to human laughter.

But you can also tickle meerkats, owls, penguins, or rats.

These animals let out laugh-like noises, and most of them seem to enjoy it.

One study found that young rats love being tickled and will seek it out, but female rats find it aversive as they get older, and will start to avoid it.

This behaviour sounds pretty similar to what we see in humans.

Kids can find being tickled entertaining, but many adults don't like being tickled at all.

The involuntary laughter is uncomfortable, or even painful, an experience that's supported by brain scans.

A research group in Germany put a bunch of people in an fMRI to study the differences between ticklish laughter and voluntary laughter.

And only ticklish laughter activated the hypothalamus, which is responsible for visceral or involuntary reactions.

Also, ticklish laughter activated parts of the brain responsible for pain anticipation.

So just because you're laughing, doesn't mean you're having a good time.

It turns out, the natural and common reflex to tickling is surprisingly complicated, and individual opinions on the experience are surprisingly varied.

Are you ticklish?

Do you like being tickled?