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

This joke is greeted with groans that resonate through a warehouse in the capital.

This describes a joke-testing session with a firm that makes products for gatherings. Its catalogue includes festive crackers.

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

"You measure the joke by the volume of groans and the loudness 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 per se. It is entirely about the context - in this case, the shared laughter of the Christmas meal with elders, children and potentially friends.

"You want the gag to be something that unites the child in harmony with the 80-year-old," she adds.

The Neuroscience Behind Shared Laughter

Gathering to enjoy communal laughter is not only nothing new, scientists argue, it is probably to be older than humanity.

"So when you are laughing with others at the Christmas dinner you are dropping into what's almost certainly a really ancient mammal play vocalisation," explains a neuroscience expert.

Communal laughter, she explains, helps make and maintain social bonds between people.

Scientists have found that a absence of such social exchanges can significantly damage both psychological and bodily health.

"Those you talk to, and share laughter with, it leads to enhanced levels of endorphin uptake," she continues.

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate tension and discomfort and in response to enjoyable activities, such as laughing with loved ones over a truly awful Christmas cracker joke.

"It's not simply chuckling at a foolish joke with a Christmas cracker," she states. "You are in fact doing a lot of the truly vital task of making, maintaining the connections you have with the people you love."

What Occurs In the Brain?

But what is actually taking place inside the brain when we listen to a joke?

A tremendous amount occurs in reaction to humour, it transpires.

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

Testing involves imaging the brains of volunteer subjects and then exposing them to a collection of humorous phrases, paired with either a non-emotional sound, or recorded laughter.

"In the scanner we observed a really fascinating pattern of neural activity," notes the neuroscientist.

A joke stimulates not just the parts of the brain in charge of hearing and understanding language, but also brain areas involved in both planning and initiating motion and those involved in vision and recall.

Combine these elements as a whole, and individuals hearing a joke have a sophisticated set of neural responses that support the laughter we hear.

The Contagious Power of Laughter

Scientists found that when a funny phrase is combined with laughter there is a greater response in the brain than the identical phrase when accompanied by a non-emotional sound.

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

It means people are not just reacting to humorous words, they are responding to the laughter that follows them.

Amusement, according to the professor, can be contagious.

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

"People laugh harder when you are familiar with others," she notes, "and you laugh more when you are fond of them or love them."

When it comes to Christmas cracker puns, she explains, the positive effect is more likely to be caused not by the gag in itself, but from the response to it.

"The laughter is key. The gag is the dreadful Christmas cracker joke, and it's just a pretext to chuckle as a group."

The Quest for the Ideal Festive Pun

Is it possible to discover the perfect joke?

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

Years ago, a psychologist set up a research project for the world's funniest gag.

More than 40,000 gags submitted, with scores provided by 350,000 people around the world, he has a better understanding than many as to what works and what does not.

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

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

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

"The reason is that if nobody laughs – it's the gag's shortcoming, not your own.

"What's interesting about the holiday cracker jokes is that not one person find them funny.

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