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

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

We're at a joke-testing meeting with a company that produces products for gatherings. Its repertoire features festive crackers.

The company's founder smiles, almost sheepishly at the joke. But the joke has been selected and will feature in future crackers.

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

The key to a great Christmas cracker pun is not the same as a stand-up gag in itself. It is entirely about the setting - in this case, the communal laughter of the Christmas dinner table with grandparents, kids and possibly friends.

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

The Science Behind Shared Laughter

Coming together to experience communal amusement is not only ancient, scientists say, it is likely to be pre-human.

"Therefore when you are laughing with others at the Christmas table you are dropping into what's almost certainly a really ancient mammalian social vocalisation," says a professor.

Communal laughter, she says, aids in make and maintain social bonds between people.

Scientists have found that a lack of these social exchanges can significantly damage both psychological and bodily well-being.

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

Endorphins are the body's "happy chemicals" and are released both to alleviate stress and pain and in response to pleasurable activities, such as chuckling with loved ones over a particularly awful festive cracker gag.

"You're not just laughing at a foolish pun with a Christmas cracker," the expert says. "You are actually doing a lot of the truly important task of making, maintaining the connections you have with those you care about."

What Happens In the Brain?

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

An awful lot happens in reaction to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which parts of the brain are working harder, researchers have been able to chart the regions that get more blood.

Testing entails imaging the minds of volunteer subjects and then subjecting them to a collection of humorous words, paired with either a neutral sound, or recorded laughter.

"In the scanner we got a really interesting pattern of activation," says the neuroscientist.

A gag stimulates not just the parts of the mind in charge of auditory processing and interpreting language, but also brain regions associated with both preparation and initiating motion and those linked to sight and memory.

Put these elements together, and individuals hearing a joke have a complex set of neural reactions that support the amusement we experience.

The Contagious Power of Laughter

Scientists discovered that when a funny word is paired with laughter there is a greater response in the mind than the identical phrase when followed by a non-emotional sound.

"This was in parts of the mind that you would employ to move your expression into a smile or a chuckle," the professor says.

It indicates we are not just responding to funny jokes, they are reacting to the amusement that follows them.

Amusement, according to the professor, can be contagious.

So what does this imply for the laughter found at a holiday gathering?

"People laugh harder when you know others," she says, "and you laugh more when you like them or love them."

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

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

The Search for the Perfect Festive Pun

Will we ever discover the ultimate gag?

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

In 2001, a psychologist set up a scientific search for the world's funniest gag.

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

The ideal Christmas cracker pun must be brief, he explains.

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

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

"The reason is that if no-one laughs – it's the joke's shortcoming, not yours.

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

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