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"How much did Santa's sleigh cost? Nothing, it was on the house."

This quip is met by moans that resonate through a warehouse in the capital.

We're at a humor-evaluation meeting with a company that makes supplies for social events. Its catalogue includes Christmas crackers.

The company's owner grins, nearly sheepishly at the joke. But the joke has been selected and will appear in future crackers.

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

The secret to a good Christmas cracker joke is not the identical as a good gag in itself. It is entirely about the setting - in this case, the communal laughter of the holiday meal with elders, children and potentially friends.

"The goal is for the gag to be a thing that brings the child together with the grandparent," she adds.

The Science Behind Shared Laughter

Gathering to experience communal amusement is not only nothing new, experts argue, it is likely to be pre-human.

"So when you are chuckling with others around the holiday table you are dropping into what's almost certainly a truly primordial mammal play vocalisation," explains a neuroscience expert.

Shared laughter, she explains, helps make and maintain social bonds between individuals.

Researchers have discovered that a lack of these interactions can seriously damage both psychological and bodily health.

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

Endorphins are the body's "feel-good compounds" and are produced both to reduce stress and pain and in response to pleasurable experiences, such as laughing with loved ones over a truly awful festive cracker gag.

"You're not just laughing at a foolish joke with a holiday cracker," she says. "You are in fact performing a lot of the really important work of making, maintaining the social bonds you have with those you love."

Which Happens In the Brain?

But what is actually happening within the brain when we listen to a gag?

A tremendous amount happens in reaction to humour, it transpires.

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which areas of the mind are more active, scientists have been able to map the regions that get more blood.

Testing entails scanning the brains of volunteer participants and then subjecting them to a collection of humorous words, accompanied by either a non-emotional sound, or recorded laughter.

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

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

Put all of this together, and people hearing a pun have a complex set of brain responses that underpin the laughter we hear.

The Infectious Nature of Laughter

Scientists discovered that when a funny word is combined with laughter there is a greater response in the mind than the identical word when accompanied by a non-emotional sound.

"This was in parts of the mind that you would use to contort your expression into a smile or a chuckle," the professor explains.

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

Laughter, according to the professor, can be contagious.

So what does this imply for the chuckles found around a Christmas gathering?

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

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

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

The Search for the Ideal Cracker Joke

Will we ever find the perfect joke?

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

In 2001, a professor established a research project for the planet's most humorous gag.

Over tens of thousands of gags submitted, with scores lodged by hundreds of thousands of people globally, he has a clearer understanding than many as to what succeeds and what does not.

The ideal Christmas cracker joke needs to be short, he explains.

"They must also be bad jokes, puns that make us groan," he adds.

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

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

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

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