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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 London.

This describes a joke-testing meeting with a company that makes supplies for gatherings. Its catalogue includes festive crackers.

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

"You measure the gag by the volume of moans and the intensity of the groans at the table," she says.

The secret to a great Christmas cracker pun is not the identical as a stand-up gag per se. It is all about the context - in this instance, the shared laughter of the Christmas meal with grandparents, children and potentially neighbours.

"You want the joke to be a thing that unites the child together with the 80-year-old," she states.

The Neuroscience Of Shared Amusement

Coming together to enjoy shared amusement is not only ancient, experts say, it is likely to be older than humanity.

"So when you are laughing with people around the holiday dinner you are engaging in what's almost certainly a truly primordial mammalian social sound," explains a neuroscience expert.

Communal laughter, she says, helps make and maintain social connections between people.

Researchers have discovered that a lack of such social exchanges can seriously harm mental and physical health.

"Those you talk to, and share laughter with, it results in increased levels of 'happy chemical' uptake," she continues.

These natural chemicals are the body's "feel-good compounds" and are produced both to alleviate tension and discomfort and in response to pleasurable activities, such as chuckling with friends over a truly awful festive cracker gag.

"It's not simply chuckling at a silly pun with a holiday cracker," she states. "You are in fact performing a lot of the truly important work of building, preserving the social bonds you have with the people you care about."

What Happens Inside the Mind?

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

An awful lot happens in response to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which parts of the mind are working harder, researchers have been able to chart the areas that get more blood.

The research involves scanning the minds of volunteer subjects and then subjecting them to a collection of funny words, paired with either a neutral sound, or pre-recorded laughter.

"During the study we observed a very interesting pattern of neural activity," notes the neuroscientist.

A gag activates not just the parts of the mind responsible for hearing and understanding language, but also brain regions associated with both preparation and initiating motion and those linked to vision and recall.

Combine all of this as a whole, and individuals hearing a pun have a complex series of brain responses that underpin the laughter we hear.

The Infectious Power of Laughter

Scientists discovered that when a funny phrase is combined with chuckles there is a greater reaction in the brain than the identical phrase when accompanied by a neutral sound.

"This was in areas of the mind that you would use to move your face into a smile or a laugh," the professor explains.

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

Laughter, says the professor, can be contagious.

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

"You laugh more when you are familiar with people," she notes, "and laughter increases further when you are fond of them or love them."

When it comes to Christmas cracker puns, she explains, the feel-good effect is more probable to be caused not by the joke in itself, but from the reaction to it.

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

The Search for the Ideal Cracker Joke

Is it possible to discover the perfect joke?

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

In 2001, a psychologist set up a scientific project for the planet's funniest joke.

Over tens of thousands of jokes later, with scores provided by hundreds of thousands of participants around the world, he has a better understanding than most as to what succeeds and what does not.

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

"But they also need to be poor jokes, puns that cause us to moan," he adds.

The more "awful" the gag, he states the more effective.

"The reason is that if nobody laughs – it's the gag's fault, not yours.

"What's interesting about the holiday cracker jokes is that none of us find them humorous.

"That's a shared moment at the table and I think it's wonderful."