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

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

We're at a humor-evaluation meeting with a company that produces supplies for gatherings. Its catalogue includes Christmas crackers.

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

"The success is gauged by the joke by the number of groans and the intensity of the groans around the table," she says.

The key to a good holiday cracker joke is not the identical as a good joke in itself. It is entirely about the setting - in this case, the shared laughter of the Christmas dinner table with elders, children and potentially neighbours.

"You want the gag to be something that unites the child in harmony with the 80-year-old," she adds.

The Neuroscience Behind Communal Laughter

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

"Therefore when you are laughing with people around the holiday dinner you are dropping into what's almost certainly a really primordial mammal play vocalisation," says a neuroscience expert.

Shared amusement, she says, helps forge and strengthen social bonds between people.

Researchers have found that a absence of these social exchanges can significantly damage both psychological and bodily health.

"The people you converse with, and laugh with, it leads to enhanced levels of 'happy chemical' release," the professor adds.

These natural chemicals are the body's "happy chemicals" and are released both to reduce stress and pain and in response to pleasurable experiences, such as chuckling with friends over a truly terrible festive cracker gag.

"You're not just chuckling at a foolish pun with a holiday cracker," she says. "You are in fact performing a lot of the really important task of making, maintaining the social bonds you have with those you care about."

What Occurs Inside the Mind?

But what is truly happening within the brain when we hear a joke?

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

Employing brain scanning technology, a type of brain scanner which shows which parts of the mind are working harder, researchers have been able to map the regions that get more blood flow.

Testing entails scanning the brains of volunteer subjects and then subjecting them to a database of humorous words, accompanied by either a non-emotional sound, or pre-recorded laughter.

"In the scanner we got a very interesting activation pattern of activation," notes the neuroscientist.

A gag activates not just the parts of the brain in charge of hearing and understanding language, but also brain regions involved in both planning and starting movement and those linked to sight and memory.

Combine all of this together, and individuals hearing a joke have a sophisticated set of neural responses that underpin the amusement we experience.

The Infectious Nature of Laughter

Researchers discovered that when a humorous phrase is combined with laughter there is a stronger reaction in the mind than the same word when accompanied by a neutral sound.

"This was in parts of the brain that you would employ to move your expression into a smile or a chuckle," the professor says.

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

Laughter, according to the expert, can be contagious.

So what does this mean for the laughter heard at a Christmas gathering?

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

When it comes to festive cracker jokes, she says, the feel-good factor is more likely to be triggered not by the gag in itself, but from the response to it.

"It's the laughter. The joke is the dreadful holiday cracker joke, and it's just a pretext to chuckle as a group."

The Quest for the Perfect Festive Pun

Will we ever discover the perfect joke?

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

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

More than tens of thousands of gags later, with ratings provided by 350,000 participants around the world, he has a clearer idea than many as to what works and what does not.

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

"They must also need to be bad jokes, jokes that make us groan," he continues.

The increasingly "terrible" the joke, he states the better.

"This is because 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 considers them funny.

"It creates a common experience at the table and I believe it's lovely."