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

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

We're at a joke-testing meeting with a company that makes supplies for social events. Its repertoire includes festive crackers.

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

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

The key to a great Christmas cracker joke is not the identical as a stand-up gag in itself. It is all about the context - in this instance, the shared laughter of the holiday dinner table with elders, children and potentially friends.

"The goal is for the gag to be a thing that brings the child together with the 80-year-old," she states.

The Neuroscience Of Communal Laughter

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

"Therefore when you are laughing with others around the holiday table you are engaging in what's almost certainly a really ancient mammal social vocalisation," says a neuroscience expert.

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

Researchers have discovered that a absence of these interactions can significantly harm both psychological and bodily well-being.

"Those you converse with, and share laughter with, it results in enhanced levels of 'happy chemical' uptake," the professor continues.

Endorphins are the brain's "feel-good compounds" and are released both to reduce stress and pain and in response to pleasurable activities, such as laughing with loved ones over a particularly terrible festive cracker joke.

"It's not simply chuckling at a foolish joke with a Christmas cracker," she states. "You are actually performing a lot of the really vital work of making, maintaining the social bonds you have with those you love."

What Occurs Inside the Brain?

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

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

Employing functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which parts of the mind are more active, researchers have been able to chart the regions that get more blood flow.

Testing involves imaging the brains of healthy participants and then subjecting them to a collection of humorous phrases, accompanied by either a neutral sound, or pre-recorded chuckles.

"During the study we observed a very interesting activation pattern of neural activity," notes the professor.

A gag stimulates not just the areas of the mind in charge of auditory processing and understanding speech, but also neural areas associated with both preparation and starting motion and those involved in vision and recall.

Put these elements as a whole, and people hearing a joke have a sophisticated set of neural responses that support the amusement we experience.

The Infectious Power of Chuckles

Researchers found that when a funny word is combined with chuckles there is a stronger response in the brain than the identical word when followed by a neutral sound.

"This was in parts of the mind that you would employ to contort your expression into a grin or a chuckle," she says.

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

Amusement, according to the professor, can be infectious.

So what does this imply for the chuckles heard around a Christmas gathering?

"You laugh more when you are familiar with people," she says, "and you laugh more when you are fond of them or care for them."

When it comes to Christmas cracker puns, she says, the feel-good effect is more likely to be triggered not by the gag itself, but from the response to it.

"It's the laughter. The gag is the dreadful Christmas cracker pun, and it's just a pretext to laugh together."

The Search for the Perfect Cracker Joke

Is it possible to find the ultimate gag?

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

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

More than tens of thousands of gags submitted, with scores lodged by 350,000 participants around the world, he has a better idea than most as to what works and what does not.

The ideal Christmas cracker joke needs to be brief, he says.

"But they also be poor jokes, puns that make us moan," he adds.

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

"The reason is that if no-one laughs – it's the joke's fault, not yours.

"What's interesting about the holiday cracker puns is that none of us find them funny.

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