This is Hidden Brain, I'm Shankar Vedantam.

When Sophie Scott was about six, she came across her parents doing something very strange.

They were rolling around, laughing.

In my memory, they were actually on the floor of the living room.

Absolutely overcome with laughter.

They loved so much, they could literally do nothing else but laugh.

What had got them laughing was a song.

A comedy song about what people were not supposed to do in toilets on trains.

It's said to be quite a famous piece of music and it's going to be custom as well.

Please refrain from passing water while the train is in the station,

giling by love.

We encourage constipation while the train is in the station.

And as they remembered more and more of it, they got more and more helpless.

If you wish to pass some water, finally call a moment later, you'll place a best song in the best song.

Now maybe you're thinking, wait, this song isn't that funny.

Try the platform in the rear.

But we've all been there, right?

You're with a friend, they say something ridiculous.

And then your laughter triggers their laughter.

Oh my god, you're good.

Sophie didn't know then that laughter would play a big role in her life.

But today she thinks about laughter a lot.

...