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

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

We're at a joke-testing session with a company that makes products for gatherings. Its catalogue includes festive crackers.

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

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

The secret to a good Christmas cracker pun is not the same as a stand-up joke in itself. It is all about the context - in this case, the communal laughter of the Christmas meal with grandparents, children and potentially neighbours.

"The goal is for the joke to be something that unites the eight-year-old in harmony with the 80-year-old," she adds.

The Science Of Shared Amusement

Coming together to experience shared amusement is not only nothing new, scientists say, it is probably to be older than humanity.

"Therefore when you are laughing with people at the holiday table you are dropping into what's very likely a truly primordial mammalian play sound," says a neuroscience expert.

Communal laughter, she explains, helps forge and strengthen social connections between people.

Scientists have found that a absence of such social exchanges can significantly damage mental and physical well-being.

"The people you converse with, and laugh with, it results in enhanced amounts of 'happy chemical' uptake," she continues.

Endorphins are the body's "happy chemicals" and are released both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as laughing with loved ones over a particularly terrible Christmas cracker joke.

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

What Happens Inside the Brain?

But what is truly happening inside the mind when we hear a gag?

A tremendous amount occurs in reaction to comedy, it turns out.

Employing brain scanning technology, a kind of neural imager which indicates which parts of the brain are working harder, researchers have been able to chart the areas that get more blood.

Testing involves scanning the minds of healthy participants and then exposing them to a collection of humorous words, accompanied by either a neutral sound, or recorded laughter.

"In the scanner we got a very interesting pattern of neural activity," notes the professor.

A joke stimulates not just the areas of the brain responsible for auditory processing and understanding speech, but also neural areas associated with both planning and initiating movement and those involved in sight and recall.

Combine all of this together, and individuals listening to a pun have a sophisticated series of brain reactions that underpin the amusement we hear.

The Infectious Nature of Laughter

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

"This was in areas of the brain that you would employ to move your face into a grin or a laugh," the professor explains.

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

Amusement, according to the professor, can be infectious.

So what does this imply for the laughter heard around a holiday table?

"People laugh harder when you know others," she says, "and laughter increases further when you like 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 joke, and it's just a reason to chuckle as a group."

The Search for the Ideal Festive Pun

Is it possible to discover the perfect joke?

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

In 2001, a professor established a scientific project for the world's most humorous joke.

Over tens of thousands of gags later, with scores provided by 350,000 people globally, he has a clearer idea than many as to what succeeds and what fails.

The ideal Christmas cracker joke needs to be brief, he says.

"But they also be bad gags, jokes that make us groan," he continues.

The increasingly "awful" the gag, he states the more effective.

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

"The fascinating part about the holiday cracker jokes is that none of us find them funny.

"It creates a shared moment at the table and I think it's wonderful."