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

This joke is greeted with groans that echo through a warehouse in the capital.

We're at a humor-evaluation meeting with a firm that makes products for social events. Its repertoire features Christmas crackers.

The firm's founder grins, almost apologetically at the gag. But the pun has made the cut and will appear in future crackers.

"You measure the joke by the number of groans and the intensity of the groans around the table," the founder says.

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

"You want the gag to be something that brings the eight-year-old in harmony with the grandparent," she states.

The Neuroscience Behind Shared Laughter

Gathering to enjoy communal amusement is not only ancient, scientists argue, it is likely to be older than humanity.

"Therefore when you are laughing with others around the holiday table you are dropping into what's very likely a really primordial mammalian play sound," says a professor.

Shared laughter, she explains, aids in make and maintain social connections between individuals.

Scientists have discovered that a lack of these interactions can significantly harm both psychological and bodily health.

"Those you converse with, and share laughter with, it leads to enhanced levels of 'happy chemical' release," the professor continues.

Endorphins are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as laughing with loved ones over a particularly awful Christmas cracker joke.

"You're not just chuckling at a foolish pun with a Christmas cracker," the expert states. "You are in fact doing a lot of the truly important task of making, maintaining the connections you have with those you love."

Which Occurs In the Mind?

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

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

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which areas of the brain are working harder, scientists have been able to map the regions that get more blood flow.

Testing involves imaging the minds of healthy subjects and then subjecting them to a collection of funny words, accompanied by either a neutral sound, or pre-recorded chuckles.

"During the study we observed a really interesting pattern of neural activity," notes the professor.

A joke activates not just the areas of the mind in charge of auditory processing and understanding speech, but also neural areas involved in both preparation and starting motion and those involved in vision and recall.

Put all of this together, and people hearing a pun have a complex set of neural responses that support the amusement we experience.

The Contagious Power of Laughter

Scientists found that when a funny phrase is combined with laughter there is a stronger response in the brain than the identical word when accompanied by a neutral sound.

"This was in areas of the mind that you would employ to contort your face into a grin or a chuckle," the professor explains.

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

Amusement, says the professor, can be contagious.

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

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

When it comes to festive cracker jokes, she says, the positive effect is more likely to be caused not by the gag in itself, but from the response to it.

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

The Search for the Ideal Festive Pun

Is it possible to find the perfect gag?

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

Years ago, a psychologist set up a scientific search for the world's funniest gag.

Over tens of thousands of jokes submitted, with scores lodged by 350,000 participants globally, he has a better idea than many as to what works and what does not.

The perfect festive cracker joke must be short, he explains.

"But they also be poor gags, jokes that cause us to groan," he adds.

The increasingly "awful" the joke, he states the better.

"This is because if nobody laughs – it's the joke's fault, not your own.

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

"That's a shared experience at the table and I think it's wonderful."