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"How much did Father Christmas's sleigh 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 firm that produces supplies for gatherings. Its repertoire includes Christmas crackers.

The company's founder grins, almost sheepishly at the gag. But the pun has made the cut and will appear in upcoming crackers.

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

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

"You want the gag to be a thing that unites the child together with the grandparent," she adds.

The Neuroscience Behind Communal Laughter

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

"So when you are chuckling with others around the holiday dinner you are dropping into what's almost certainly a really primordial mammalian play sound," says a neuroscience expert.

Communal laughter, she says, helps make and maintain social bonds between individuals.

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

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

Endorphins are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in response to enjoyable activities, such as laughing with loved ones over a truly terrible festive cracker joke.

"It's not simply laughing at a silly pun with a Christmas cracker," the expert states. "You are in fact performing a lot of the really vital work of building, preserving the connections you have with the people you love."

What Occurs Inside the Brain?

But what is actually taking place within the mind when we hear a joke?

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

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

Testing involves scanning the brains of volunteer participants and then exposing them to a database of funny words, accompanied by either a non-emotional sound, or recorded laughter.

"In the scanner we observed a really interesting activation pattern of activation," notes the professor.

A gag activates not just the areas of the mind responsible for hearing and understanding language, but also neural areas associated with both preparation and starting movement and those involved in vision and memory.

Put these elements together, and individuals listening to a pun have a complex set of neural reactions that underpin the laughter we experience.

The Infectious Power of Laughter

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

"This was in parts of the brain that you would use to contort your expression into a grin or a chuckle," she explains.

It indicates we are not just reacting to funny words, they are responding to the laughter that follows them.

Amusement, according to the professor, can be infectious.

So what does this imply for the laughter found at a Christmas gathering?

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

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

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

The Quest for the Perfect Cracker Joke

Will we ever discover the perfect gag?

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

Years ago, a professor set up a research project for the planet's funniest joke.

More than 40,000 gags later, with ratings provided by 350,000 participants around the world, he has a better idea than many as to what succeeds and what fails.

The perfect festive cracker joke must be brief, he says.

"But they also be poor gags, puns that cause us to moan," he continues.

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

"This is because if nobody laughs – it's the gag's fault, not your own.

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

"That's a shared experience at the table and I think it's lovely."