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

This quip is greeted with groans that resonate through a warehouse in the capital.

We're at a joke-testing session with a company that produces supplies for gatherings. Its catalogue includes festive crackers.

The company's owner smiles, nearly sheepishly at the joke. But the pun has been selected and will appear in future crackers.

"You measure the joke by the number of moans and the loudness of the groans at the table," the founder says.

The key to a good holiday cracker joke is not the identical as a good joke in itself. It is entirely about the context - in this instance, the shared laughter of the holiday meal with elders, children and potentially neighbours.

"You want the joke to be something that unites the child together with the grandparent," she adds.

The Neuroscience Behind Communal Laughter

Gathering to enjoy communal amusement is not only ancient, experts say, it is probably to be pre-human.

"So when you are chuckling with others around the holiday dinner you are dropping into what's very likely a truly primordial mammal play vocalisation," says a neuroscience expert.

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

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

"Those you converse with, and laugh with, it leads to increased amounts of 'happy chemical' uptake," she adds.

These natural chemicals are the body's "happy chemicals" and are released both to alleviate stress and pain and in response to pleasurable experiences, such as laughing with friends over a particularly terrible Christmas cracker joke.

"It's not simply laughing at a silly pun with a Christmas cracker," she says. "You are in fact doing a lot of the truly important task of making, maintaining the connections you have with those you care about."

What Happens Inside the Brain?

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

An awful lot occurs in response to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which areas of the brain are working harder, scientists have been able to chart the regions that get more blood.

The research involves scanning the brains of volunteer subjects and then exposing them to a collection of humorous words, accompanied by either a neutral sound, or pre-recorded laughter.

"During the study we got a really interesting pattern of activation," notes the neuroscientist.

A joke activates not just the parts of the mind in charge of auditory processing and interpreting speech, but also neural areas associated with both preparation and starting motion and those linked to vision and recall.

Combine these elements as a whole, and individuals listening to a joke have a sophisticated series of brain responses that support the amusement we experience.

The Contagious Nature of Laughter

Scientists found that when a humorous word is paired with laughter there is a greater response in the mind than the same word when followed by a non-emotional sound.

"This was in parts of the brain that you would employ to move your face into a grin or a laugh," the professor explains.

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

Laughter, says the expert, can be infectious.

So what does this imply for the chuckles heard at a holiday table?

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

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

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

The Search for the Perfect Cracker Joke

Will we ever discover the ultimate gag?

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

In 2001, a professor established a research project for the planet's funniest joke.

More than 40,000 gags submitted, with ratings provided by hundreds of thousands of participants 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 groan," he continues.

The more "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 puns is that not one person considers them funny.

"That's a shared experience at the gathering and I think it's lovely."