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

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

We're at a joke-testing meeting with a firm that produces products for social events. Its repertoire features Christmas crackers.

The company's owner grins, nearly apologetically at the joke. But the joke has made the cut and will feature in future crackers.

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

The secret to a good holiday cracker joke is not the identical as a good gag per se. It is all about the context - in this case, the communal laughter of the Christmas meal with grandparents, kids and potentially friends.

"The goal is for the gag to be something that unites the eight-year-old in harmony with the grandparent," she adds.

The Neuroscience Behind Communal Laughter

Coming together to enjoy communal laughter is not only ancient, scientists argue, it is probably to be pre-human.

"Therefore when you are laughing with people around the Christmas table you are engaging in what's almost certainly a truly primordial mammal play sound," explains a neuroscience expert.

Shared amusement, she says, helps make and maintain social connections between individuals.

Scientists have discovered that a absence of such interactions can significantly harm both psychological and bodily health.

"The people you talk to, and laugh with, it results in 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 tension and discomfort and in reaction to pleasurable activities, such as laughing with friends over a truly awful festive cracker gag.

"It's not simply chuckling at a foolish joke with a Christmas cracker," the expert says. "You are in fact performing a lot of the truly important work of building, preserving the connections you have with the people you care about."

Which Occurs Inside the Brain?

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

A tremendous amount occurs in response to humour, it transpires.

Employing brain scanning technology, a type of brain scanner which indicates which parts of the mind are more active, researchers have been able to chart the regions that receive more blood.

The research involves scanning the brains of volunteer participants and then subjecting them to a collection of humorous words, paired with either a non-emotional sound, or pre-recorded chuckles.

"During the study we got a very fascinating pattern of activation," notes the professor.

A joke activates not just the parts of the mind in charge of hearing and interpreting language, but also neural areas involved in both planning and starting movement and those linked to vision and memory.

Put these elements as a whole, and people listening to a pun have a complex series of brain reactions that support the amusement we hear.

The Infectious Power of Chuckles

Scientists found that when a humorous word is combined with chuckles there is a stronger reaction in the mind than the identical word when followed by a non-emotional sound.

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

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

Amusement, according to the expert, can be infectious.

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

"You laugh harder when you are familiar with people," she notes, "and you laugh more when you are fond of them or love them."

When it comes to Christmas cracker puns, she explains, the feel-good factor is more probable to be triggered not by the gag in itself, but from the reaction to it.

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

The Quest for the Perfect Festive Pun

Will we ever find the perfect gag?

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

In 2001, a psychologist established a scientific project for the planet's funniest gag.

More than 40,000 gags later, with scores lodged by hundreds of thousands of participants around the world, he has a clearer understanding than many as to what succeeds and what fails.

The perfect Christmas cracker pun must be short, he explains.

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

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

"This is because if nobody laughs – it's the gag's shortcoming, not your own.

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

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