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

This one-liner is greeted with moans that echo through a storage facility in the capital.

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

The firm's owner smiles, almost apologetically at the gag. But the joke has made the cut and will feature in future crackers.

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

The key to a great Christmas cracker joke is not the identical as a stand-up joke per se. It is entirely about the context - in this case, the communal laughter of the Christmas dinner table with elders, kids and possibly neighbours.

"You want the joke to be a thing that brings the eight-year-old together with the grandparent," she adds.

The Neuroscience Of Shared Laughter

Coming together to enjoy communal amusement is not only nothing new, experts say, it is probably to be older than humanity.

"Therefore when you are laughing with people around the holiday table you are engaging in what's almost certainly a really ancient mammal social sound," says a professor.

Shared laughter, she explains, helps make and maintain social connections between people.

Scientists have discovered that a lack of these social exchanges can significantly damage mental and physical well-being.

"The people you converse with, and share laughter with, it leads to enhanced amounts of endorphin release," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce tension and discomfort and in reaction to pleasurable activities, such as laughing with friends over a particularly awful Christmas cracker gag.

"It's not simply laughing at a foolish joke with a holiday cracker," she says. "You are actually doing a lot of the really important task of building, preserving the social bonds you have with the people you love."

Which Happens Inside the Brain?

But what is truly happening within the brain when we listen to a joke?

A tremendous amount happens in response to comedy, it transpires.

Employing brain scanning technology, a kind of brain scanner which indicates which parts of the brain are more active, researchers have been able to chart the areas that receive more blood flow.

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

"During the study we got a really fascinating pattern of neural activity," notes the neuroscientist.

A joke activates not just the parts of the mind responsible for hearing and understanding language, but also brain areas associated with both preparation and initiating motion and those linked to sight and memory.

Put all of this together, and individuals listening to a pun have a sophisticated set of brain responses that support the amusement we experience.

The Contagious Power of Laughter

Researchers discovered that when a funny phrase is paired with laughter there is a stronger reaction in the mind than the identical phrase when accompanied by a neutral sound.

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

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

Laughter, says the expert, can be infectious.

So what does this mean for the laughter found at a holiday gathering?

"People laugh harder when you are familiar with people," she says, "and you laugh further when you are fond of them or care for them."

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

"The laughter is key. The gag is the terrible Christmas cracker joke, and it's just a reason to laugh as a group."

The Quest for the Ideal 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 project for the planet's most humorous joke.

More than tens of thousands of jokes later, with scores lodged by 350,000 people globally, he has a clearer idea than many as to what works and what fails.

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

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

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

"This is because if no-one laughs – it's the joke's shortcoming, not your own.

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

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