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

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

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

The firm's founder smiles, nearly sheepishly at the joke. But the pun has been selected and will feature in upcoming crackers.

"You measure the joke by the volume of groans and the intensity of the groans at the table," she says.

The secret to a good Christmas cracker pun is not the identical as a good joke in itself. It is entirely about the context - in this case, the shared laughter of the Christmas meal with grandparents, children and potentially neighbours.

"You want the joke to be a thing that unites the eight-year-old in harmony with the 80-year-old," she adds.

The Neuroscience Behind Communal Laughter

Coming together to experience shared amusement is not only ancient, experts argue, it is probably to be pre-human.

"So when you are chuckling with others around the holiday table you are dropping into what's almost certainly a really primordial mammalian social vocalisation," explains a neuroscience expert.

Communal laughter, she explains, helps forge and strengthen social bonds between individuals.

Scientists have discovered that a lack of these interactions can significantly damage mental and physical health.

"Those you converse with, and share laughter with, it leads to enhanced levels of 'happy chemical' uptake," she adds.

These natural chemicals are the body's "happy chemicals" and are released both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as laughing with loved ones over a truly awful Christmas cracker gag.

"You're not just laughing at a foolish joke with a holiday cracker," the expert 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."

Which Happens In the Brain?

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

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

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

The research involves imaging the brains of healthy subjects and then subjecting them to a database of humorous phrases, accompanied by either a non-emotional sound, or recorded laughter.

"During the study we got a really fascinating activation pattern of activation," notes the professor.

A gag stimulates not just the parts of the mind responsible for auditory processing and interpreting speech, but also neural areas associated with both planning and starting motion and those involved in sight and recall.

Combine these elements as a whole, and people hearing a joke have a sophisticated set of neural responses that underpin the amusement we hear.

The Infectious Power of Laughter

Researchers found that when a funny word is combined with chuckles there is a stronger reaction in the mind than the identical phrase when accompanied by a neutral sound.

"This was in areas of the brain that you would use to move your face into a smile or a chuckle," she explains.

It means people are not just reacting to funny words, they are reacting to the amusement that follows them.

Laughter, says the professor, can be contagious.

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

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

When it comes to Christmas cracker jokes, she explains, the feel-good effect is more probable to be caused not by the joke itself, but from the reaction to it.

"It's the laughter. The joke is the terrible Christmas cracker joke, and it's just a pretext to chuckle as a group."

The Search for the Perfect Festive Pun

Will we ever discover the ultimate joke?

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

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

Over tens of thousands of jokes submitted, with ratings provided by 350,000 participants around the world, he has a clearer idea than most as to what succeeds and what fails.

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

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

The increasingly "terrible" the joke, he states the more effective.

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

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

"That's a shared experience around the gathering and I think it's wonderful."