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"How much did Santa'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.

We're at a humor-evaluation meeting with a firm that produces products for gatherings. Its catalogue features Christmas crackers.

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

"The success is gauged by the gag by the number of groans and the intensity of the groans at the table," the founder explains.

The secret to a great holiday cracker pun is not the same as a good gag in itself. It is entirely about the setting - in this instance, the shared amusement of the Christmas meal with elders, children and potentially neighbours.

"The goal is for the gag to be something that unites the child in harmony with the grandparent," she states.

The Neuroscience Behind Shared Amusement

Coming together to enjoy communal amusement is not only nothing new, experts say, it is probably to be pre-human.

"Therefore when you are laughing with people at the Christmas dinner you are dropping into what's very likely a truly primordial mammal social vocalisation," explains a neuroscience expert.

Communal amusement, she explains, helps make and maintain social bonds between people.

Researchers have found that a absence of these social exchanges can significantly damage mental and physical well-being.

"Those you converse with, and share laughter with, it leads to increased levels of 'happy chemical' uptake," the professor continues.

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

"It's not simply laughing at a foolish pun with a Christmas cracker," she says. "You are actually doing a lot of the really important task of making, maintaining the social bonds you have with those you love."

What Happens In the Brain?

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

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

Employing brain scanning technology, a kind of brain scanner which indicates which parts of the mind are more active, researchers have been able to chart the areas that receive more blood flow.

Testing entails imaging the minds of volunteer participants and then exposing them to a database of funny phrases, accompanied by either a non-emotional sound, or pre-recorded chuckles.

"In the scanner we observed a very fascinating activation pattern of neural activity," says the professor.

A gag activates not just the areas of the brain in charge of auditory processing and interpreting speech, but also brain areas associated with both planning and initiating motion and those linked to vision and memory.

Combine these elements together, and individuals hearing a pun have a sophisticated set of brain reactions that underpin the amusement we experience.

The Contagious Nature of Laughter

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

"This was in areas of the brain that you would employ to move your expression into a smile or a laugh," the professor explains.

It indicates we are not just reacting to funny jokes, they are reacting to the laughter that accompanies them.

Amusement, according to the professor, can be infectious.

So what does this imply for the laughter heard around a Christmas table?

"You laugh more when you are familiar with others," she says, "and you laugh more when you are fond of them or love them."

When it comes to festive cracker jokes, she says, the positive factor is more probable to be triggered not by the gag in itself, but from the reaction to it.

"The laughter is key. The joke is the dreadful holiday cracker pun, and it's just a pretext to chuckle together."

The Quest for the Ideal Festive Pun

Is it possible to find the ultimate joke?

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

In 2001, a psychologist set up a scientific search for the world's funniest joke.

Over 40,000 gags submitted, with scores lodged by hundreds of thousands of participants globally, he has a clearer idea than many as to what works and what fails.

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

"But they also need to be bad jokes, jokes that cause us to moan," he continues.

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

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

"The fascinating part about the Christmas cracker jokes is that not one person considers them humorous.

"It creates a shared experience at the gathering and I think it's wonderful."