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

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

This describes a joke-testing session with a firm that produces supplies for social events. Its catalogue includes festive crackers.

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

"The success is gauged by the gag by the number of moans and the loudness of the groans at the table," she explains.

The secret to a great Christmas cracker joke is not the same as a good joke per se. It is entirely about the context - in this instance, the communal laughter of the holiday meal with elders, children and possibly neighbours.

"You want the joke to be something that unites the eight-year-old together with the grandparent," she states.

The Science Of Shared Laughter

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

"Therefore when you are chuckling with people at the holiday dinner you are engaging in what's almost certainly a truly ancient mammalian play vocalisation," explains a professor.

Shared amusement, she says, helps make and maintain social bonds between people.

Scientists have discovered that a lack of these interactions can seriously damage mental and physical well-being.

"The people you converse with, and laugh with, it leads to enhanced levels of endorphin uptake," she continues.

Endorphins are the brain's "happy chemicals" and are released both to alleviate tension and discomfort and in response to enjoyable activities, such as chuckling with loved ones over a particularly terrible festive cracker gag.

"You're not just chuckling at a silly pun with a holiday cracker," the expert says. "You are actually performing a lot of the really vital task of making, maintaining the social bonds you have with those you care about."

Which Happens In the Mind?

But what is truly taking place within the brain when we hear a gag?

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

Using brain scanning technology, a kind of brain scanner which indicates which parts of the mind are more active, scientists have been able to map the regions that receive more blood flow.

The research involves imaging the brains of volunteer subjects and then exposing them to a database of funny phrases, paired with either a neutral sound, or recorded laughter.

"During the study we observed a really interesting activation pattern of activation," notes the professor.

A gag stimulates not just the areas of the mind in charge of auditory processing and understanding language, but also neural regions involved in both planning and starting motion and those involved in sight and memory.

Combine these elements together, and people hearing a pun have a complex set of brain reactions that underpin the laughter we experience.

The Contagious Power of Laughter

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

"This was in areas of the brain that you would use to contort your expression into a grin or a laugh," she says.

It indicates we are not just responding to humorous words, they are reacting to the amusement that accompanies them.

Amusement, according to the expert, can be contagious.

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

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

When it comes to festive cracker puns, she says, the feel-good factor is more likely to be triggered not by the joke 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 reason to chuckle as a group."

The Search for the Ideal Cracker Joke

Is it possible to find the perfect gag?

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

Years ago, a professor established a research project for the world's most humorous gag.

More than 40,000 gags later, with ratings lodged by hundreds of thousands of people around the world, he has a clearer idea than most as to what works and what fails.

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

"But they also need to be poor jokes, puns that make us groan," he adds.

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

"The reason is that if no-one finds it funny – it's the joke's shortcoming, not yours.

"What's interesting about the Christmas cracker puns is that not one person considers them funny.

"It creates a common moment at the gathering and I believe it's lovely."