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

This one-liner is greeted with moans that resonate through a warehouse in the capital.

This describes a joke-testing meeting with a firm that produces products for gatherings. Its repertoire includes Christmas crackers.

The firm's owner grins, almost apologetically at the gag. But the joke has been selected and will appear in upcoming crackers.

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

The key to a good Christmas cracker pun is not the identical as a stand-up joke in itself. It is entirely about the setting - in this instance, the shared amusement of the Christmas meal with grandparents, children and possibly friends.

"The goal is for the joke to be something that brings the child together with the grandparent," she adds.

The Neuroscience Behind Shared Laughter

Gathering to experience shared laughter is not only ancient, experts say, it is probably to be older than humanity.

"Therefore when you are chuckling with others at the Christmas dinner you are engaging in what's very likely a truly ancient mammal play vocalisation," says a professor.

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

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

"The people you converse with, and share laughter with, it results in increased levels of 'happy chemical' release," she continues.

Endorphins are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in response to pleasurable experiences, such as laughing with loved ones over a particularly terrible festive cracker gag.

"It's not simply chuckling at a silly joke with a Christmas cracker," she states. "You are in fact doing a lot of the really vital work of building, preserving the social bonds you have with those you care about."

Which Occurs Inside the Brain?

But what is truly happening within the brain when we hear a gag?

A tremendous amount occurs in response to humour, it turns out.

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 map the areas that receive more blood.

Testing entails scanning the minds of healthy subjects and then subjecting them to a collection of humorous words, paired with either a non-emotional sound, or pre-recorded laughter.

"In the scanner we got a really fascinating pattern of neural activity," notes the professor.

A gag stimulates not just the parts of the mind in charge of auditory processing and understanding speech, but also brain regions involved in both preparation and initiating movement and those involved in sight and memory.

Put these elements together, and people listening to a pun have a complex set of neural responses that support the laughter we experience.

The Contagious Nature of Chuckles

Researchers discovered that when a funny word is combined with laughter there is a greater reaction in the mind than the same word when accompanied by a neutral sound.

"This activation occurred in parts of the mind that you would use to contort your face into a grin or a laugh," the professor says.

It indicates we are not just reacting to funny jokes, they are responding to the laughter that accompanies them.

Amusement, says the expert, can be infectious.

So what does this mean for the chuckles heard around a holiday table?

"You laugh more when you are familiar with others," she says, "and laughter increases more when you like them or care for them."

When it comes to festive cracker puns, she explains, the positive factor is more probable to be caused not by the joke itself, but from the response to it.

"It's the laughter. The joke is the dreadful holiday cracker pun, and it's just a pretext to laugh together."

The Search for the Perfect Cracker Joke

Will we ever find the ultimate gag?

Probably not, but that has not stopped researchers from attempting to.

In 2001, a professor set up a research search for the planet's funniest joke.

Over tens of thousands of jokes later, with scores provided by hundreds of thousands of participants globally, he has a better idea than most as to what succeeds and what fails.

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

"But they also be poor jokes, jokes that cause us to moan," he continues.

The increasingly "terrible" the joke, he says the better.

"The reason is that if nobody finds it funny – it's the joke's shortcoming, not yours.

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

"It creates a shared experience around the table and I believe it's wonderful."