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"How much did Santa's sleigh cost? Zero, it was on the house."

This joke is met by groans that resonate through a storage facility in the capital.

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

The company's owner smiles, nearly apologetically at the gag. But the joke has made the cut and will feature in upcoming crackers.

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

The secret to a great Christmas cracker joke is not the same as a stand-up gag in itself. It is entirely about the context - in this case, the shared amusement of the holiday meal with grandparents, kids and potentially friends.

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

The Neuroscience Of Communal Laughter

Gathering to experience communal amusement is not only nothing new, experts say, it is probably to be older than humanity.

"So when you are laughing with people at the Christmas table you are engaging in what's almost certainly a really ancient mammal social sound," explains a professor.

Communal laughter, she says, helps forge and strengthen social bonds between people.

Scientists have discovered that a lack of these social exchanges can significantly harm both psychological and bodily well-being.

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

These natural chemicals are the body's "happy chemicals" and are released both to alleviate tension and discomfort and in response to pleasurable activities, such as laughing with loved ones over a truly terrible Christmas cracker joke.

"It's not simply chuckling at a foolish pun with a Christmas cracker," she states. "You are in fact performing a lot of the truly important work of making, maintaining the social bonds you have with the people you love."

What Happens Inside the Brain?

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

A tremendous amount happens in response to humour, it turns out.

Employing functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which parts of the mind are working harder, researchers have been able to chart the areas that receive more blood flow.

The research entails imaging the minds of volunteer participants and then subjecting them to a collection of funny words, paired with either a neutral sound, or recorded chuckles.

"In the scanner we got a very interesting pattern of neural activity," notes the professor.

A joke activates not just the parts of the mind responsible for auditory processing and interpreting speech, but also neural regions involved in both planning and starting movement and those linked to sight and recall.

Put all of this as a whole, and individuals listening to a joke have a sophisticated series of brain responses that underpin the laughter we hear.

The Infectious Nature of Laughter

Researchers discovered that when a funny word is paired with laughter there is a stronger response in the mind than the identical phrase when accompanied by a non-emotional sound.

"This activation occurred in areas of the mind that you would employ to contort your expression into a grin or a chuckle," the professor explains.

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

Laughter, says the professor, can be contagious.

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

"You laugh harder when you know people," she notes, "and laughter increases further when you like them or love them."

When it comes to Christmas cracker jokes, she says, the feel-good factor is more likely to be caused not by the gag in itself, but from the response to it.

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

The Search for the Ideal Festive Pun

Is it possible to discover the perfect gag?

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

Years ago, a psychologist set up a scientific project for the world's funniest joke.

More than tens of thousands of jokes later, with scores lodged by 350,000 people around the world, he has a better understanding than many as to what works and what fails.

The ideal Christmas cracker pun must be brief, he says.

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

The more "terrible" the gag, he says the more effective.

"This is because if no-one finds it funny – it's the gag's fault, not yours.

"What's interesting about the Christmas cracker puns is that none of us considers them funny.

"It creates a shared moment at the gathering and I believe it's wonderful."