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

This one-liner is met by groans that echo through a warehouse in the capital.

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

The company's owner smiles, almost apologetically at the gag. But the joke has made the cut and will feature in upcoming crackers.

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

The key to a good holiday cracker joke is not the identical as a good joke per se. It is all about the setting - in this instance, the shared laughter of the Christmas meal with grandparents, children and possibly friends.

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

The Science Behind Shared Laughter

Coming together to experience communal amusement is not only ancient, 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 primordial mammalian play vocalisation," explains a professor.

Communal amusement, she explains, aids in make and maintain social bonds between individuals.

Scientists have found that a lack of such interactions can significantly damage mental and physical well-being.

"The people you talk to, and laugh with, it leads to enhanced amounts of 'happy chemical' uptake," the professor adds.

These natural chemicals are the body's "feel-good compounds" and are released both to alleviate stress and pain and in reaction to enjoyable experiences, such as laughing with loved ones over a particularly terrible Christmas cracker joke.

"You're not just chuckling at a silly joke with a Christmas cracker," she says. "You are actually performing a lot of the really important task of making, maintaining the social bonds you have with those you care about."

What Happens Inside the Mind?

But what is actually happening within the brain when we listen to a gag?

An awful lot happens in reaction to humour, it transpires.

Employing brain scanning technology, a kind of neural imager which indicates which parts of the mind are more active, researchers have been able to map the regions that receive more blood flow.

Testing entails imaging the minds of healthy subjects and then exposing them to a database of humorous words, paired with either a non-emotional sound, or pre-recorded chuckles.

"During the study we observed a very fascinating pattern of neural activity," says the professor.

A gag activates not just the parts of the brain responsible for hearing and interpreting language, but also brain regions involved in both planning and initiating movement and those linked to sight and memory.

Combine all of this together, and individuals listening to a pun have a sophisticated series of neural reactions that underpin the amusement we hear.

The Contagious Power of Laughter

Researchers discovered that when a humorous word is paired with chuckles there is a greater response in the mind than the same word when accompanied by a non-emotional sound.

"This activation occurred in parts of the mind that you would use to contort your face into a smile or a laugh," she says.

It means people are not just responding to humorous words, they are reacting to the amusement that follows them.

Amusement, says the professor, can be contagious.

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

"You laugh more when you are familiar with others," she says, "and laughter increases more when you are fond of them or love them."

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

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

The Quest for the Ideal Cracker Joke

Will we ever find the perfect gag?

Likely not, but that has not stopped experts from trying to.

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

Over 40,000 gags submitted, with ratings provided by hundreds of thousands of participants around the world, he has a better understanding than many as to what succeeds and what does not.

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

"But they also be bad jokes, jokes that cause us to groan," he continues.

The increasingly "awful" the joke, he says the more effective.

"The reason is that if nobody laughs – it's the gag's fault, not yours.

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

"It creates a shared experience around the table and I think it's lovely."