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"How much did Santa's sleigh cost? Nothing, it was on the house."

This one-liner is met by groans that echo through a storage facility in London.

We're at a joke-testing session with a company that produces products for social events. Its catalogue features Christmas crackers.

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

"The success is gauged by the joke by the number of groans and the loudness of the groans at the table," the founder explains.

The key to a great Christmas cracker pun is not the same as a stand-up joke in itself. It is all about the context - in this instance, the communal amusement of the holiday meal with elders, kids and possibly neighbours.

"The goal is for the gag to be a thing that brings the child in harmony with the grandparent," she adds.

The Science Behind Shared Amusement

Coming together to experience shared amusement is not only ancient, experts argue, it is probably to be pre-human.

"So when you are laughing with people around the Christmas dinner you are dropping into what's almost certainly a really primordial mammal social vocalisation," says a neuroscience expert.

Communal amusement, she explains, aids in forge and strengthen social bonds between people.

Researchers have discovered that a lack of such interactions can seriously damage mental and physical well-being.

"The people you talk to, and laugh with, it leads to increased levels of 'happy chemical' release," she continues.

Endorphins are the body's "happy chemicals" and are released both to reduce tension and discomfort and in response to pleasurable experiences, such as laughing with friends over a truly terrible festive cracker gag.

"It's not simply laughing at a foolish pun with a Christmas cracker," the expert states. "You are in fact doing a lot of the really vital task of building, preserving the social bonds you have with the people you love."

Which Occurs In the Mind?

But what is truly taking place within the brain when we listen to a gag?

An awful lot happens in response to humour, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which parts of the brain are working harder, researchers have been able to chart the regions that receive more blood flow.

The research involves imaging the minds of healthy subjects and then exposing them to a database of funny phrases, paired with either a neutral sound, or pre-recorded chuckles.

"In the scanner we got a really interesting activation pattern of neural activity," says the neuroscientist.

A joke activates not just the areas of the mind in charge of auditory processing and interpreting language, but also neural regions involved in both preparation and starting motion and those involved in vision and memory.

Put these elements as a whole, and people hearing a joke have a sophisticated set of brain reactions that support the laughter we experience.

The Contagious Power of Chuckles

Scientists discovered that when a humorous word is paired with chuckles there is a stronger reaction in the brain than the same phrase when accompanied by a neutral sound.

"This activation occurred in parts of the mind that you would use to contort your expression into a grin or a laugh," she explains.

It means we are not just responding to humorous jokes, they are reacting to the amusement that follows them.

Laughter, according to the expert, can be contagious.

So what does this mean for the laughter found at a Christmas gathering?

"People laugh more when you are familiar with others," she says, "and laughter increases further when you are fond of them or love them."

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

"The laughter is key. The joke is the terrible Christmas cracker pun, and it's just a reason to laugh as a group."

The Quest for the Ideal Cracker Joke

Will we ever discover the ultimate joke?

Probably not, but that has not prevented researchers from trying to.

In 2001, a professor established a research project for the world's most humorous gag.

More than tens of thousands of gags submitted, with scores lodged by 350,000 participants globally, he has a clearer understanding than most as to what succeeds and what fails.

The ideal festive cracker joke needs to be short, he explains.

"But they also need to be bad jokes, jokes that cause us to moan," he adds.

The increasingly "terrible" the joke, he says the more effective.

"The reason is that if no-one laughs – it's the joke's shortcoming, not yours.

"What's interesting about the Christmas cracker puns is that not one person find them humorous.

"It creates a shared experience at the table and I think it's lovely."