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"What was the price did Father Christmas's sleigh cost? Nothing, it was on the house."

This one-liner is met by groans that echo through a warehouse in the capital.

This describes a joke-testing session with a firm that makes products for social events. Its repertoire features Christmas crackers.

The company's founder smiles, almost apologetically at the gag. But the joke has been selected and will feature in future crackers.

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

The key to a good holiday cracker joke is not the identical as a good joke in itself. It is all about the context - in this instance, the shared laughter of the holiday meal with grandparents, children and possibly friends.

"The goal is for the gag to be a thing that brings the eight-year-old together with the 80-year-old," she adds.

The Science Behind Shared Amusement

Gathering to experience communal laughter is not only ancient, experts say, it is probably to be pre-human.

"Therefore when you are chuckling with others at the holiday table you are engaging in what's very likely a truly primordial mammal play vocalisation," says a neuroscience expert.

Shared laughter, she says, helps forge and strengthen social connections between people.

Researchers have discovered that a lack of these social exchanges can seriously harm both psychological and bodily health.

"Those you converse with, and laugh with, it results in increased levels of 'happy chemical' uptake," she continues.

Endorphins are the body's "feel-good compounds" and are produced both to reduce stress and pain and in reaction to pleasurable activities, such as chuckling with friends over a particularly terrible Christmas cracker gag.

"It's not simply laughing at a silly pun with a Christmas cracker," she says. "You are actually performing a lot of the really vital task of making, maintaining the connections you have with those you care about."

What Happens Inside the Brain?

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

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

Employing brain scanning technology, a kind of brain scanner which indicates which parts of the mind are more active, researchers have been able to map the regions that get more blood.

Testing involves imaging the minds of healthy subjects and then exposing them to a collection of funny words, paired with either a neutral sound, or pre-recorded chuckles.

"During the study we got a very fascinating pattern of neural activity," says the neuroscientist.

A gag stimulates not just the parts of the brain in charge of auditory processing and interpreting language, but also neural regions associated with both planning and initiating movement and those involved in vision and memory.

Put these elements together, and individuals hearing a joke have a sophisticated set of neural responses that underpin the laughter we experience.

The Contagious Power of Laughter

Scientists found that when a funny phrase is combined with chuckles there is a greater reaction in the mind than the identical phrase when followed by a neutral sound.

"This activation occurred in parts of the brain that you would use to move your expression into a grin or a chuckle," the professor says.

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

Laughter, according to the expert, can be infectious.

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

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

When it comes to festive cracker puns, she explains, the positive effect is more probable to be triggered not by the joke in itself, but from the reaction to it.

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

The Quest for the Ideal Cracker Joke

Is it possible to find the perfect gag?

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

Years ago, a psychologist set up a scientific search for the world's most humorous joke.

More than tens of thousands of jokes later, with scores provided by hundreds of thousands of people globally, he has a clearer understanding than many as to what works and what does not.

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

"They must also need to be bad jokes, puns that cause us to groan," he continues.

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

"The reason is that if no-one finds it funny – it's the gag's fault, not yours.

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

"It creates a common moment around the table and I think it's lovely."