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"How much did Santa's sled cost? Zero, it was on the house."

This quip is greeted with groans that echo through a warehouse in the capital.

This describes a joke-testing meeting with a firm that produces products for social events. Its catalogue includes festive crackers.

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

"The success is gauged by the joke by the volume of moans and the intensity of the groans around the table," she says.

The secret to a good Christmas cracker pun is not the same as a good joke in itself. It is all about the setting - in this instance, the communal amusement of the holiday dinner table with grandparents, children and potentially neighbours.

"You want the joke to be something that brings the eight-year-old in harmony with the 80-year-old," she adds.

The Science Of Communal Laughter

Gathering to enjoy shared amusement is not only nothing new, experts argue, it is likely to be older than humanity.

"So when you are laughing with people around the holiday table you are engaging in what's very likely a really ancient mammal play vocalisation," explains a professor.

Communal laughter, she says, aids in make and maintain social connections between individuals.

Researchers have discovered that a absence of these social exchanges can significantly harm both psychological and bodily well-being.

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

Endorphins are the brain's "happy chemicals" and are produced both to alleviate tension and discomfort and in response to enjoyable experiences, such as laughing with friends over a particularly terrible Christmas cracker joke.

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

What Occurs In the Mind?

But what is truly happening within the mind when we listen to a gag?

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

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which areas of the brain are more active, researchers have been able to map the areas that get more blood flow.

Testing involves scanning the minds of volunteer subjects and then exposing them to a database of funny words, accompanied by either a non-emotional sound, or pre-recorded laughter.

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

A gag activates not just the parts of the brain in charge of hearing and interpreting language, but also brain regions associated with both preparation and starting movement and those linked to sight and memory.

Combine all of this together, and people hearing a joke have a sophisticated set of neural responses that support the laughter we experience.

The Contagious Power of Laughter

Scientists found that when a humorous word is combined with chuckles there is a stronger response in the brain than the same word when accompanied by a non-emotional sound.

"This was in parts of the mind that you would use to move your expression into a grin or a laugh," she says.

It means we are not just responding to funny words, they are reacting to the amusement that accompanies them.

Laughter, according to the professor, can be infectious.

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

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

When it comes to Christmas cracker jokes, she says, the positive effect is more likely to be triggered not by the gag itself, but from the response to it.

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

The Search for the Perfect Cracker Joke

Is it possible to discover the perfect gag?

Likely not, but that has not prevented experts from trying to.

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

Over 40,000 jokes submitted, with scores lodged by hundreds of thousands of people around the world, he has a better understanding than most as to what works and what fails.

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

"But they also need to be poor gags, jokes that make us groan," he continues.

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

"This is because if no-one laughs – it's the gag's shortcoming, not yours.

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

"It creates a common moment at the table and I believe it's lovely."