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"How much did Father Christmas's sleigh cost? Zero, it was on the house."

This joke is greeted with moans that echo through a warehouse in London.

We're at a humor-evaluation meeting with a company that produces products for social events. Its catalogue includes Christmas crackers.

The company's founder grins, almost apologetically at the gag. But the pun has made the cut and will feature in future crackers.

"The success is gauged by the gag by the number of moans and the loudness of the groans at the table," the founder says.

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

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

The Neuroscience Of Communal Laughter

Gathering to enjoy shared amusement is not only ancient, experts argue, it is probably to be older than humanity.

"Therefore when you are chuckling with others at the holiday dinner you are engaging in what's very likely a really ancient mammalian play sound," explains a professor.

Shared laughter, she explains, helps forge and strengthen social bonds between people.

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

"Those you talk to, and laugh with, it leads to enhanced levels of 'happy chemical' release," the professor continues.

Endorphins are the body's "feel-good compounds" and are released both to reduce tension and discomfort and in reaction to pleasurable experiences, such as laughing with friends over a particularly terrible Christmas cracker joke.

"It's not simply chuckling at a silly joke with a holiday cracker," she says. "You are actually performing a lot of the truly vital task of building, preserving the social bonds you have with the people you care about."

Which Occurs In the Mind?

But what is actually happening inside the mind when we listen to a gag?

An awful lot occurs in reaction to humour, it transpires.

Using brain scanning technology, a kind of brain scanner which shows which parts of the mind are working harder, researchers have been able to map the regions that receive more blood flow.

The research involves scanning the brains of healthy subjects and then subjecting them to a database of funny phrases, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we observed a really interesting activation pattern of activation," says the professor.

A joke stimulates not just the parts of the brain in charge of hearing and understanding language, but also neural regions associated with both planning and starting movement and those linked to vision and memory.

Put all of this as a whole, and individuals hearing a joke have a sophisticated set of brain reactions that underpin the amusement we hear.

The Contagious Power of Chuckles

Scientists found that when a humorous word is combined with chuckles there is a stronger reaction in the brain than the same phrase when followed by a neutral sound.

"This was in parts of the mind that you would employ to contort your face into a grin or a laugh," she says.

It means we are not just responding to humorous words, they are responding to the laughter that accompanies them.

Laughter, according to the professor, can be contagious.

So what does this imply for the chuckles heard at a holiday gathering?

"You laugh more when you know people," she says, "and you laugh further when you are fond of them or care for them."

When it comes to festive cracker jokes, she says, the positive factor is more likely to be caused not by the joke in itself, but from the response to it.

"It's the laughter. The gag is the terrible holiday cracker pun, and it's just a reason to chuckle together."

The Search for the Perfect Cracker Joke

Is it possible to discover the perfect gag?

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

In 2001, a professor set up a research search for the world's most humorous gag.

Over 40,000 jokes submitted, with scores lodged by hundreds of thousands of participants globally, he has a clearer understanding than many as to what succeeds and what fails.

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

"They must also be bad jokes, puns that make us moan," he continues.

The more "terrible" the joke, he states the more effective.

"This is because if no-one laughs – it's the gag's fault, not your own.

"The fascinating part about the Christmas cracker puns is that not one person considers them funny.

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