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

This one-liner is met by moans that resonate through a warehouse in the capital.

This describes a joke-testing session with a firm that makes products for gatherings. Its repertoire includes festive crackers.

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

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

The key to a good holiday cracker joke is not the same as a stand-up joke per se. It is all about the setting - in this instance, the communal amusement of the holiday dinner table with grandparents, children and potentially friends.

"The goal is for the joke to be something that brings the child in harmony with the grandparent," she adds.

The Neuroscience Behind Shared Amusement

Gathering to experience communal laughter is not only ancient, scientists argue, it is likely to be older than humanity.

"Therefore when you are laughing with others around the Christmas dinner you are dropping into what's very likely a really primordial mammalian social vocalisation," says a professor.

Shared laughter, she explains, helps forge and strengthen social connections between individuals.

Researchers have discovered that a absence of such interactions can seriously damage mental and physical well-being.

"The people you talk to, and share laughter with, it leads to enhanced amounts of 'happy chemical' uptake," she continues.

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce tension and discomfort and in reaction to pleasurable experiences, such as chuckling with loved ones over a particularly awful Christmas cracker gag.

"It's not simply laughing at a silly pun with a Christmas cracker," she states. "You are actually performing a lot of the really important work of making, maintaining the social bonds you have with the people you love."

Which Occurs In the Brain?

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

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

Employing brain scanning technology, a kind of neural imager which shows which areas of the brain are working harder, researchers have been able to chart the regions that receive more blood.

The research involves imaging the minds of volunteer participants and then exposing them to a database of funny phrases, paired with either a non-emotional sound, or pre-recorded laughter.

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

A joke stimulates not just the areas of the mind responsible for auditory processing and interpreting language, but also brain areas involved in both preparation and initiating motion and those involved in sight and memory.

Combine these elements as a whole, and people hearing a joke have a sophisticated set of brain reactions that support the amusement we experience.

The Contagious Power of Laughter

Scientists found that when a humorous phrase is paired with laughter 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 move your face into a smile or a laugh," the professor explains.

It means we are not just responding to funny words, they are responding to the amusement that follows them.

Laughter, according to the expert, can be infectious.

So what does this mean for the laughter found at a holiday table?

"You laugh harder when you know others," she says, "and laughter increases more when you are fond of them or care for them."

When it comes to Christmas cracker jokes, she explains, the positive factor is more likely to be triggered not by the gag in itself, but from the response to it.

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

The Search for the Ideal Festive Pun

Is it possible to discover the ultimate joke?

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

Years ago, a psychologist set up a scientific search for the planet's most humorous joke.

More than tens of thousands of gags later, with scores lodged by hundreds of thousands of participants globally, he has a better idea than most as to what succeeds and what does not.

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

"They must also be poor gags, puns that make us moan," he adds.

The increasingly "terrible" the gag, he says the better.

"This is because if no-one laughs – it's the joke's shortcoming, not your own.

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

"That's a common moment at the gathering and I think it's lovely."