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

This quip is met by moans that resonate through a warehouse in the capital.

This describes a humor-evaluation meeting with a firm that produces supplies for social events. Its repertoire features Christmas crackers.

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

"You measure the gag by the volume of groans and the loudness of the groans around the table," she explains.

The key to a good Christmas cracker joke is not the identical as a stand-up joke per se. It is all about the context - in this instance, the communal amusement of the holiday dinner table with grandparents, children and possibly friends.

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

The Neuroscience Of Shared Amusement

Gathering to enjoy communal laughter is not only nothing new, experts argue, it is probably to be older than humanity.

"Therefore when you are chuckling with people around the Christmas table you are engaging in what's very likely a truly ancient mammal social sound," explains a professor.

Shared laughter, she explains, helps make and maintain social bonds between people.

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

"The people you converse with, and laugh with, it leads to increased levels of 'happy chemical' uptake," she continues.

Endorphins are the body's "feel-good compounds" and are produced both to alleviate tension and discomfort and in response to pleasurable activities, such as laughing with friends over a particularly awful festive cracker gag.

"It's not simply chuckling at a silly joke with a holiday cracker," she states. "You are actually doing a lot of the truly important task of building, preserving the social bonds you have with the people you love."

What Happens Inside the Brain?

But what is actually taking place within the mind when we hear a joke?

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

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which shows which parts of the mind are working harder, scientists have been able to chart the regions that get more blood flow.

Testing involves imaging the brains of healthy subjects and then subjecting them to a database of funny words, accompanied by either a non-emotional sound, or recorded laughter.

"During the study we observed a very fascinating activation pattern of activation," says the neuroscientist.

A gag activates not just the areas of the brain responsible for hearing and understanding language, but also neural areas involved in both planning and starting motion and those linked to sight and memory.

Put these elements as a whole, and individuals listening to a pun have a sophisticated series of brain responses that underpin the amusement we experience.

The Infectious Nature of Laughter

Researchers found that when a funny phrase is paired with chuckles there is a stronger response in the brain than the same phrase when followed by a neutral sound.

"This was in parts of the brain that you would use to contort your face into a grin or a laugh," the professor explains.

It indicates people are not just responding to humorous words, they are reacting to the amusement that follows them.

Amusement, says the expert, can be infectious.

So what does this mean for the chuckles heard at a holiday table?

"You laugh harder when you know people," she notes, "and you laugh further when you are fond of them or care for them."

When it comes to Christmas cracker jokes, she says, the feel-good factor is more likely to be caused not by the joke in 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 chuckle together."

The Quest for the Perfect Festive Pun

Is it possible to discover the ultimate gag?

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

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

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

The perfect Christmas cracker joke needs to be short, he says.

"But they also be bad gags, jokes that make us moan," he continues.

The more "awful" the gag, he states the more effective.

"This is because if no-one finds it funny – it's the joke's shortcoming, not your own.

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

"It creates a common moment at the table and I believe it's wonderful."