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

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

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

The company's owner grins, nearly sheepishly at the gag. But the joke has been selected and will appear in future crackers.

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

The key to a great Christmas cracker joke is not the identical as a stand-up joke per se. It is entirely about the context - in this case, the communal laughter of the Christmas meal with grandparents, children and potentially friends.

"You want the gag to be something that brings the child in harmony with the grandparent," she adds.

The Neuroscience Of Communal Amusement

Coming together to experience communal laughter is not only nothing new, experts argue, it is likely to be older than humanity.

"Therefore when you are laughing with others around the Christmas table you are engaging in what's very likely a truly primordial mammalian social sound," says a professor.

Communal amusement, she says, helps forge and strengthen social bonds between individuals.

Researchers have found that a absence of these social exchanges can seriously damage mental and physical health.

"The people you talk to, and share laughter with, it results in increased amounts of 'happy chemical' uptake," the professor adds.

These natural chemicals are the body's "feel-good compounds" and are produced both to reduce stress and pain and in reaction to pleasurable activities, such as laughing with friends over a truly terrible festive cracker gag.

"It's not simply chuckling at a silly pun with a holiday cracker," she states. "You are in fact doing a lot of the really vital work of building, preserving the social bonds you have with the people you care about."

Which Happens Inside the Mind?

But what is actually happening within the mind when we listen to a joke?

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

Using brain scanning technology, a kind of neural imager which shows which parts of the brain are more active, scientists have been able to map the areas that get more blood flow.

The research entails scanning the minds of healthy subjects and then subjecting them to a collection of humorous phrases, paired with either a neutral sound, or pre-recorded laughter.

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

A joke stimulates not just the areas of the brain in charge of hearing and interpreting language, but also brain areas involved in both preparation and starting motion and those involved in vision and recall.

Put all of this as a whole, and people listening to a pun have a complex series of brain responses that support the laughter we experience.

The Contagious Nature of Chuckles

Researchers found that when a humorous word is combined with laughter there is a stronger response in the brain than the same phrase when followed by a neutral sound.

"This activation occurred in areas of the brain that you would use to contort your face into a smile or a chuckle," the professor says.

It means people are not just responding to funny words, they are responding to the laughter that accompanies them.

Laughter, says the expert, can be contagious.

So what does this imply for the chuckles heard around a holiday table?

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

When it comes to Christmas cracker puns, she says, the positive effect is more probable to be caused not by the joke in itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Is it possible to find the perfect joke?

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

Years ago, a professor established a research project for the world's most humorous joke.

More than 40,000 jokes later, with scores lodged by hundreds of thousands of people globally, he has a better idea than most as to what works and what does not.

The ideal festive cracker pun needs to be brief, he says.

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

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

"The reason is that if nobody laughs – it's the gag's shortcoming, not your own.

"The fascinating part 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."