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"What was the price did Santa's sleigh cost? Nothing, it was on the house."

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

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

The firm's founder grins, nearly sheepishly at the joke. But the joke has been selected and will feature in upcoming crackers.

"You measure the gag by the number of groans and the loudness of the groans around the table," she says.

The secret to a good Christmas cracker joke is not the same as a stand-up joke per se. It is all about the context - in this case, the communal laughter of the holiday dinner table with grandparents, children and possibly neighbours.

"The goal is for the gag to be something that brings the eight-year-old in harmony with the 80-year-old," she adds.

The Science Of Shared Laughter

Coming together to experience communal amusement is not only nothing new, scientists argue, it is probably to be pre-human.

"So when you are chuckling with people around the Christmas dinner you are dropping into what's very likely a really ancient mammalian social sound," explains a neuroscience expert.

Communal laughter, she explains, aids in make and maintain social connections between people.

Scientists have discovered that a lack of such interactions can seriously harm mental and physical health.

"The people you converse with, and laugh with, it results in increased amounts of 'happy chemical' release," she adds.

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in reaction to pleasurable experiences, such as laughing with friends over a truly awful Christmas cracker joke.

"You're not just chuckling at a silly joke with a holiday cracker," she states. "You are in fact performing a lot of the really vital task of building, preserving the social bonds you have with those you care about."

Which Happens Inside the Mind?

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

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

Using brain scanning technology, a kind of neural imager which shows which areas of the mind are working harder, scientists have been able to map the areas that receive more blood flow.

Testing entails imaging the minds of healthy participants and then subjecting them to a database of funny words, accompanied by either a neutral sound, or recorded laughter.

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

A joke stimulates 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.

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

The Contagious Nature of Chuckles

Researchers discovered that when a humorous word is combined with laughter there is a greater response in the mind than the identical phrase when accompanied by a neutral sound.

"This activation occurred in areas of the mind that you would employ to contort your face into a smile or a chuckle," she explains.

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

Laughter, according to the professor, can be infectious.

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

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

When it comes to Christmas cracker jokes, she says, the positive factor is more probable to be caused not by the gag itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the perfect joke?

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

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

Over 40,000 jokes submitted, with ratings lodged by hundreds of thousands of participants around the world, he has a better idea than most as to what succeeds and what fails.

The ideal Christmas cracker pun needs to be brief, he explains.

"But they also need to be bad gags, jokes that cause us to moan," he adds.

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

"The reason is that if no-one laughs – it's the gag's shortcoming, not your own.

"The fascinating part about the holiday cracker jokes is that not one person find them funny.

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