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"What was the price did Santa's sled cost? Nothing, it was on the house."

This joke is met by groans that resonate through a storage facility in London.

We're at a joke-testing meeting with a firm that makes supplies for gatherings. Its repertoire includes Christmas crackers.

The firm's founder grins, nearly sheepishly at the joke. But the joke has been selected and will feature in future crackers.

"You measure the gag by the number of moans and the loudness of the groans around the table," she says.

The key to a good Christmas cracker pun is not the same as a good joke per se. It is entirely about the setting - in this case, the shared laughter of the holiday dinner table with grandparents, kids and potentially friends.

"The goal is for the joke to be a thing that unites the eight-year-old together with the grandparent," she adds.

The Neuroscience Behind Communal Laughter

Gathering to experience communal amusement is not only ancient, scientists argue, it is probably to be pre-human.

"Therefore when you are chuckling with people at the holiday dinner you are engaging in what's very likely a really primordial mammalian play sound," says a neuroscience expert.

Communal laughter, she says, aids in make and maintain social bonds between people.

Researchers have found that a lack of such interactions can seriously damage mental and physical health.

"The people you converse with, and share laughter with, it results in increased amounts of endorphin release," the professor continues.

These natural chemicals are the body's "feel-good compounds" and are released both to reduce tension and discomfort and in reaction to enjoyable experiences, such as chuckling with friends over a particularly awful Christmas cracker joke.

"You're not just laughing at a silly joke with a Christmas cracker," the expert says. "You are actually doing a lot of the really vital task of building, preserving the social bonds you have with the people you love."

Which Happens Inside the Brain?

But what is truly happening inside the brain when we listen to a joke?

An awful lot occurs in reaction to humour, it turns out.

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which parts of the brain are working harder, scientists have been able to map the regions that get more blood flow.

The research involves imaging the minds of volunteer subjects and then exposing them to a collection of humorous words, accompanied by either a non-emotional sound, or recorded chuckles.

"During the study we observed a really fascinating pattern of activation," says the neuroscientist.

A joke stimulates not just the parts of the brain responsible for auditory processing and understanding speech, but also brain regions involved in both preparation and starting movement and those linked to vision and recall.

Put all of this together, and individuals listening to a joke have a sophisticated series of brain responses that support the amusement we experience.

The Contagious Power of Laughter

Researchers discovered that when a funny phrase is combined with chuckles there is a stronger reaction in the mind than the identical phrase when accompanied by a non-emotional sound.

"This activation occurred in parts of the mind that you would use to move your face into a smile or a chuckle," she explains.

It means people are not just responding to funny jokes, they are responding to the laughter that follows them.

Laughter, says the expert, can be infectious.

So what does this mean for the chuckles heard around a Christmas gathering?

"You laugh harder when you know others," she says, "and laughter increases further when you like them or love them."

When it comes to festive cracker jokes, she explains, the positive effect is more probable to be triggered not by the gag itself, but from the reaction to it.

"It's the laughter. The gag is the terrible Christmas cracker joke, and it's just a pretext to chuckle as a group."

The Quest for the Perfect Cracker Joke

Is it possible to find the perfect gag?

Probably not, but that has not prevented experts from attempting to.

In 2001, a psychologist set up a research search for the planet's most humorous joke.

More than 40,000 gags submitted, with ratings lodged by 350,000 participants globally, he has a better idea than many as to what works and what does not.

The ideal festive cracker joke must be brief, he says.

"But they also be poor gags, puns that make us moan," he continues.

The increasingly "awful" the gag, he states the better.

"The reason is that if nobody finds it funny – it's the gag's fault, not your own.

"What's interesting about the holiday cracker jokes is that none of us find them funny.

"That's a shared experience at the gathering and I believe it's wonderful."