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

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

We're at a humor-evaluation meeting with a firm that makes supplies for social events. Its catalogue includes Christmas crackers.

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

"You measure the gag by the number of moans and the intensity of the groans at the table," she explains.

The key to a good Christmas cracker joke is not the same as a good gag per se. It is entirely about the setting - in this instance, the shared amusement of the holiday meal with elders, children and possibly friends.

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

The Neuroscience Behind Communal Laughter

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

"So when you are chuckling with people at the Christmas table you are dropping into what's very likely a really primordial mammalian social vocalisation," explains a neuroscience expert.

Communal laughter, she says, aids in make and maintain social connections between individuals.

Scientists have found that a lack of these interactions can seriously harm both psychological and bodily well-being.

"Those you talk to, and laugh with, it leads to enhanced levels of endorphin uptake," she continues.

These natural chemicals are the body's "happy chemicals" and are produced both to alleviate tension and discomfort and in reaction to enjoyable experiences, such as chuckling with friends over a particularly awful Christmas cracker gag.

"You're not just chuckling at a foolish joke with a holiday cracker," the expert says. "You are actually doing a lot of the really vital work of building, preserving the connections you have with the people you care about."

Which Occurs Inside the Brain?

But what is truly taking place within the mind when we listen to a gag?

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

Employing brain scanning technology, a type of brain scanner which shows which parts of the brain are more active, researchers have been able to map the areas that receive more blood.

Testing entails scanning the brains of volunteer participants and then subjecting them to a database of funny words, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we got a very interesting activation pattern of neural activity," says the neuroscientist.

A joke stimulates not just the areas of the brain responsible for hearing and interpreting language, but also neural areas involved in both preparation and initiating movement and those involved in sight and recall.

Put all of this as a whole, and people listening to a joke have a sophisticated set of brain responses that support the laughter we experience.

The Infectious Power of Laughter

Researchers found that when a humorous word is combined with laughter there is a greater reaction in the mind than the identical phrase when followed by a non-emotional sound.

"This was in parts of the brain that you would use to move your face into a smile or a chuckle," the professor explains.

It indicates we are not just responding to funny jokes, they are reacting to the laughter that accompanies them.

Laughter, according to the professor, can be infectious.

So what does this imply for the chuckles found at a holiday gathering?

"You laugh harder when you know others," she notes, "and laughter increases more when you are fond of them or love them."

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

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

The Quest for the Perfect Festive Pun

Will we ever find the perfect gag?

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

In 2001, a professor established a scientific search for the world's funniest joke.

More than 40,000 gags later, with scores lodged by hundreds of thousands of people around the world, he has a clearer understanding than many as to what works and what does not.

The ideal Christmas cracker joke must be short, he says.

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

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

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

"The fascinating part about the Christmas cracker jokes is that none of us considers them humorous.

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