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

This quip is met by moans that echo through a warehouse in the capital.

This describes a joke-testing session with a company that produces products for social events. Its catalogue includes Christmas crackers.

The firm's founder smiles, almost apologetically at the gag. But the pun has made the cut and will feature in upcoming crackers.

"The success is gauged by the joke by the volume of moans and the loudness of the groans at the table," she says.

The key to a good holiday cracker joke is not the same as a good gag in itself. It is entirely about the context - in this case, the communal amusement of the holiday meal with elders, children and possibly neighbours.

"You want the joke to be something that brings the eight-year-old in harmony with the 80-year-old," she adds.

The Neuroscience Of Communal Laughter

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

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

Shared laughter, she says, helps forge and strengthen social connections between individuals.

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

"Those you talk to, and laugh with, it results in increased amounts of endorphin uptake," she continues.

Endorphins are the brain's "feel-good compounds" and are produced both to reduce tension and discomfort and in response to pleasurable activities, such as chuckling with friends over a truly terrible festive cracker joke.

"It's not simply chuckling at a foolish joke with a holiday cracker," she says. "You are actually performing a lot of the really vital task of making, maintaining the social bonds you have with those you care about."

What Happens In the Brain?

But what is actually taking place within the brain when we hear a joke?

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

Employing brain scanning technology, a kind of neural imager which shows which parts of the brain are more active, researchers have been able to chart the areas that receive more blood.

Testing involves imaging the minds of volunteer subjects and then exposing them to a database of humorous phrases, accompanied by either a non-emotional sound, or pre-recorded laughter.

"During the study we observed a very fascinating activation pattern of activation," says the neuroscientist.

A gag stimulates not just the areas of the brain responsible for hearing and interpreting language, but also neural areas associated with both preparation and starting movement and those linked to vision and memory.

Combine these elements as a whole, and individuals hearing a joke have a sophisticated series of neural reactions that support the amusement we experience.

The Contagious Nature of Chuckles

Researchers found that when a humorous phrase is combined with laughter there is a greater response in the brain than the identical phrase when accompanied by a non-emotional sound.

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

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

Laughter, says the professor, can be infectious.

So what does this mean for the laughter found around a Christmas table?

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

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

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

The Search for the Perfect Cracker Joke

Will we ever find the perfect joke?

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

In 2001, a psychologist set up a scientific project for the world's most humorous joke.

Over 40,000 jokes later, with scores lodged by 350,000 participants around the world, he has a better understanding than most as to what succeeds and what fails.

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

"But they also be bad gags, puns that make us groan," he adds.

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

"The reason is that if nobody finds it funny – it's the joke's shortcoming, not yours.

"The fascinating part about the holiday cracker jokes is that none of us find them funny.

"It creates a shared moment around the table and I believe it's lovely."