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"What was the price did Santa's sled cost? Nothing, it was on the house."

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

This describes a humor-evaluation meeting with a firm that makes products for social events. Its repertoire includes Christmas crackers.

The company's owner grins, almost sheepishly at the joke. But the pun has been selected and will feature in upcoming crackers.

"You measure the gag by the volume of moans and the loudness of the groans around the table," the founder explains.

The secret to a great Christmas cracker pun is not the same as a good gag per se. It is entirely about the context - in this instance, the communal amusement of the holiday dinner table with grandparents, children and potentially neighbours.

"You want the joke to be something that brings the eight-year-old in harmony with the 80-year-old," she adds.

The Science Behind Communal Amusement

Coming together to experience shared laughter is not only ancient, scientists say, it is likely to be pre-human.

"So when you are laughing with people around the Christmas table you are engaging in what's almost certainly a really ancient mammal social sound," says a neuroscience expert.

Communal amusement, she explains, helps make and maintain social connections between individuals.

Researchers have discovered that a absence of such social exchanges can significantly harm both psychological and bodily health.

"Those you converse with, and share laughter with, it leads to enhanced levels of endorphin release," she adds.

These natural chemicals are the body's "feel-good compounds" and are released both to reduce stress and pain and in response to enjoyable experiences, such as laughing with loved ones over a truly awful festive cracker joke.

"It's not simply chuckling at a foolish joke with a Christmas cracker," the expert states. "You are in fact performing a lot of the really vital task of making, maintaining the connections you have with those you love."

What Happens Inside the Brain?

But what is actually happening inside the mind when we hear a joke?

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 regions that receive more blood flow.

Testing entails scanning the minds of healthy participants and then subjecting them to a collection of funny phrases, accompanied by either a non-emotional sound, or recorded chuckles.

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

A gag stimulates not just the parts of the mind responsible for auditory processing and interpreting speech, but also neural regions associated with both preparation and initiating movement and those linked to sight and recall.

Combine these elements as a whole, and individuals listening to a joke have a complex set of brain responses that underpin the laughter we hear.

The Contagious Power of Laughter

Researchers found that when a funny word is combined with laughter there is a greater response in the mind than the same phrase when accompanied by a neutral sound.

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

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

Laughter, says the expert, can be infectious.

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

"People laugh more when you know others," she says, "and laughter increases more when you like them or love them."

When it comes to Christmas cracker puns, she says, the feel-good factor is more probable to be caused not by the gag itself, but from the reaction to it.

"The laughter is key. The joke is the terrible Christmas cracker pun, and it's just a pretext to laugh together."

The Quest for the Ideal Festive Pun

Is it possible to discover the perfect joke?

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

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

More than 40,000 jokes later, with scores provided by 350,000 people around the world, he has a clearer idea than many as to what works and what fails.

The ideal Christmas cracker joke must be short, he says.

"But they also be poor gags, jokes that make us moan," he continues.

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

"This is because if no-one laughs – it's the gag's shortcoming, not yours.

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

"It creates a common experience at the gathering and I think it's lovely."