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

This quip is greeted with moans that echo through a storage facility in the capital.

This describes a humor-evaluation meeting with a firm that produces supplies for social events. Its catalogue features Christmas crackers.

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

"You measure the joke by the volume of groans and the intensity of the groans at the table," the founder explains.

The secret to a good Christmas cracker joke is not the identical as a good joke per se. It is all about the setting - in this case, the shared amusement of the holiday dinner table with grandparents, kids and potentially friends.

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

The Neuroscience Behind Shared Laughter

Coming together to experience shared laughter is not only ancient, scientists say, it is likely to be pre-human.

"So when you are chuckling with others around the Christmas dinner you are engaging in what's almost certainly a really primordial mammalian social vocalisation," explains a neuroscience expert.

Communal amusement, she explains, helps make and maintain social connections between people.

Scientists have discovered that a absence of these interactions can significantly harm mental and physical well-being.

"The people you converse with, and share laughter with, it results in enhanced levels of 'happy chemical' uptake," the professor continues.

Endorphins are the body's "feel-good compounds" and are released both to reduce stress and pain and in reaction to enjoyable activities, such as chuckling with loved ones over a truly terrible Christmas cracker gag.

"It's not simply chuckling at a foolish pun with a Christmas cracker," the expert says. "You are actually performing a lot of the truly vital work of making, maintaining the social bonds you have with those you care about."

Which Happens Inside the Mind?

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

An awful lot occurs in response to humour, it turns out.

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

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

"In the scanner we got a very fascinating activation pattern of neural activity," notes the neuroscientist.

A gag stimulates not just the parts of the mind in charge of auditory processing and interpreting speech, but also neural regions associated with both preparation and initiating movement and those linked to vision and memory.

Combine these elements together, and people listening to a joke have a complex set of brain responses that support the laughter we hear.

The Contagious Power of Laughter

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

"This was in parts of the mind that you would employ to move your expression into a smile or a chuckle," she says.

It means people are not just reacting to humorous jokes, they are reacting to the laughter that accompanies them.

Amusement, according to the professor, can be infectious.

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

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

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

"The laughter is key. The joke is the terrible Christmas cracker joke, and it's just a reason to chuckle together."

The Search for the Ideal Festive Pun

Is it possible to discover the perfect joke?

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

Years ago, a professor established a scientific search for the planet's funniest joke.

Over 40,000 gags later, with scores provided by 350,000 people globally, he has a clearer understanding than most as to what works and what does not.

The perfect Christmas cracker joke must be brief, he explains.

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

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

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

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

"It creates a common experience around the table and I believe it's lovely."