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

This one-liner is greeted with moans that resonate through a warehouse in the capital.

We're at a humor-evaluation meeting with a firm that produces products for gatherings. Its catalogue features Christmas crackers.

The company's founder grins, almost apologetically at the joke. But the joke has been selected and will appear in upcoming crackers.

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

The secret to a great holiday cracker joke is not the same as a good joke in itself. It is entirely about the setting - in this instance, the communal laughter of the Christmas meal with elders, children and potentially neighbours.

"You want the gag to be something that brings the child together with the 80-year-old," she states.

The Science Of Shared Amusement

Coming together to experience communal laughter is not only nothing new, experts say, it is probably to be pre-human.

"Therefore when you are laughing with people around the Christmas dinner you are dropping into what's almost certainly a truly ancient mammal social sound," explains a neuroscience expert.

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

Researchers have discovered that a absence of these social exchanges can seriously harm both psychological and bodily well-being.

"The people you talk to, and share laughter with, it results in enhanced amounts of 'happy chemical' release," she adds.

Endorphins are the brain's "happy chemicals" and are released both to reduce stress and pain and in reaction to enjoyable experiences, such as chuckling with friends over a truly terrible Christmas cracker gag.

"You're not just chuckling at a silly pun with a Christmas cracker," the expert states. "You are in fact doing a lot of the truly vital task of building, preserving the social bonds you have with those you care about."

Which Happens Inside the Mind?

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

An awful lot happens in response to comedy, it transpires.

Employing brain scanning technology, a kind of brain scanner which indicates which parts of the brain are working harder, scientists have been able to chart the regions that get more blood flow.

The research entails imaging the brains of volunteer subjects and then subjecting them to a database of funny words, paired with either a neutral sound, or recorded laughter.

"During the study we got a very interesting pattern of neural activity," says the neuroscientist.

A gag activates not just the areas of the brain responsible for auditory processing and interpreting language, but also neural areas involved in both preparation and starting motion and those linked to vision and memory.

Put these elements together, and people listening to a joke have a complex set of brain reactions that support the laughter we hear.

The Infectious Nature of Chuckles

Researchers discovered that when a funny word is paired with laughter there is a stronger reaction in the brain than the identical word when followed by a neutral sound.

"This activation occurred in areas of the mind that you would use to move your face into a grin or a laugh," the professor says.

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

Amusement, according to the professor, can be contagious.

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

"You laugh harder when you know others," she says, "and laughter increases further when you like them or love them."

When it comes to festive cracker jokes, she explains, the feel-good effect is more probable to be triggered 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 reason to chuckle as a group."

The Quest for the Ideal Festive Pun

Is it possible to find the ultimate joke?

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

Years ago, a psychologist established a research search for the world's funniest gag.

More than 40,000 jokes submitted, with scores provided by hundreds of thousands of people around the world, he has a clearer idea than most as to what works and what does not.

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

"They must also be bad jokes, jokes that cause us to groan," he continues.

The increasingly "terrible" the joke, he says the better.

"The reason is that if nobody finds it funny – it's the gag's fault, not your own.

"The fascinating part about the Christmas cracker puns is that not one person find them humorous.

"That's a common moment at the gathering and I believe it's wonderful."