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

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

This describes a humor-evaluation meeting with a company that produces products for gatherings. Its catalogue features Christmas crackers.

The firm's founder grins, nearly apologetically at the joke. But the joke has made the cut and will feature in upcoming crackers.

"The success is gauged by the joke by the number of groans and the intensity of the groans around the table," the founder says.

The key to a good Christmas cracker joke is not the identical as a good gag per se. It is entirely about the context - in this instance, the communal amusement of the holiday dinner table with elders, children and possibly neighbours.

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

The Neuroscience Of Shared Amusement

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

"So when you are chuckling with others around the holiday table you are engaging in what's almost certainly a really primordial mammalian play sound," explains a neuroscience expert.

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

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

"Those you converse with, and share laughter with, it leads to enhanced amounts of endorphin uptake," she adds.

Endorphins are the brain's "happy chemicals" and are released both to reduce stress and pain and in response to enjoyable experiences, such as laughing with friends over a particularly awful Christmas cracker gag.

"You're not just chuckling at a silly joke with a holiday cracker," the expert says. "You are in fact performing a lot of the really vital task of making, maintaining the social bonds you have with the people you care about."

Which Occurs Inside the Brain?

But what is truly taking place inside the brain when we listen to a joke?

An awful lot occurs in reaction to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which parts of the brain are more active, scientists have been able to chart the regions that get more blood.

Testing entails imaging the brains of volunteer participants and then subjecting them to a database of funny phrases, accompanied by either a neutral sound, or pre-recorded laughter.

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

A gag stimulates not just the parts of the mind responsible for auditory processing and understanding speech, but also neural areas involved in both preparation and initiating movement and those linked to vision and recall.

Put all of this together, and people hearing a pun have a sophisticated set of neural reactions that support the laughter we hear.

The Infectious Power of Chuckles

Researchers discovered that when a humorous phrase is combined with chuckles there is a greater reaction in the mind than the same phrase when accompanied by a neutral sound.

"This was in areas of the mind that you would use to contort your expression into a smile or a chuckle," the professor explains.

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

Amusement, says the professor, can be infectious.

So what does this mean for the laughter heard around a holiday gathering?

"People laugh more when you are familiar with others," she says, "and laughter increases further when you like them or love them."

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

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

The Quest for the Perfect Cracker Joke

Will we ever find the perfect gag?

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

Years ago, a professor set up a research search for the world's funniest joke.

Over 40,000 jokes submitted, with scores lodged by 350,000 people around the world, he has a clearer understanding than most as to what works and what does not.

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

"But they also need to be bad gags, puns that make us groan," he continues.

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

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

"What's interesting about the holiday cracker puns is that none of us find them humorous.

"It creates a common moment at the table and I think it's lovely."