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

This one-liner is greeted with groans that echo through a storage facility in the capital.

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

The company's founder smiles, nearly apologetically at the joke. But the pun has been selected and will feature in future crackers.

"The success is gauged by the joke by the number of groans and the loudness of the groans around the table," the founder explains.

The secret to a good holiday cracker joke is not the same as a good gag in itself. It is entirely about the context - in this case, the shared laughter of the holiday dinner table with grandparents, children and possibly neighbours.

"You want the joke to be a thing that unites the child in harmony with the 80-year-old," she states.

The Neuroscience Behind Communal Laughter

Gathering to enjoy shared laughter is not only nothing new, scientists argue, it is probably to be older than humanity.

"Therefore when you are laughing with others around the holiday table you are engaging in what's very likely a truly ancient mammalian play sound," says a neuroscience expert.

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

Scientists have discovered that a lack of these social exchanges can significantly damage mental and physical well-being.

"The people you converse with, and laugh with, it leads to increased amounts of 'happy chemical' release," the professor continues.

Endorphins are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to pleasurable experiences, such as chuckling with friends over a particularly terrible Christmas cracker gag.

"You're not just laughing at a silly pun with a holiday cracker," she says. "You are actually doing a lot of the really important work of building, preserving the connections you have with the people you love."

Which Occurs In the Mind?

But what is truly taking place inside the brain when we hear a joke?

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

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which areas of the mind are working harder, scientists have been able to chart the regions that receive more blood flow.

The research involves scanning the brains of healthy subjects and then exposing them to a collection of funny words, accompanied by either a neutral sound, or recorded chuckles.

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

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

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

The Contagious Nature of Chuckles

Scientists found that when a funny phrase is combined with chuckles there is a stronger reaction in the mind than the identical phrase when accompanied by a neutral sound.

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

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

Amusement, says the professor, can be infectious.

So what does this mean for the chuckles found at a Christmas table?

"People laugh harder when you know others," she notes, "and you laugh more when you like them or love them."

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

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

The Search for the Ideal Cracker Joke

Will we ever discover the perfect gag?

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

In 2001, a psychologist set up a scientific project for the planet's most humorous joke.

Over tens of thousands of gags later, with scores lodged by hundreds of thousands of people globally, he has a clearer understanding than most as to what succeeds and what fails.

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

"They must also be bad gags, jokes that cause us to groan," he continues.

The more "awful" the gag, he says the better.

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

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

"That's a shared experience around the gathering and I believe it's wonderful."