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"What was the price did Father Christmas's sleigh cost? Nothing, it was on the house."

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

This describes a joke-testing session with a company that produces products for gatherings. Its catalogue includes Christmas crackers.

The company's owner grins, nearly sheepishly at the joke. But the pun has made the cut and will appear in upcoming crackers.

"You measure the joke by the number of groans and the loudness of the groans around the table," the founder explains.

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

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

The Science Of Communal Laughter

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

"Therefore when you are chuckling with people around the Christmas dinner you are dropping into what's almost certainly a truly ancient mammalian social sound," says a professor.

Communal amusement, she says, helps make and maintain social bonds between people.

Researchers have discovered that a lack of these interactions can seriously harm mental and physical well-being.

"Those you talk to, and laugh with, it leads to increased levels of 'happy chemical' uptake," the professor adds.

Endorphins are the body's "feel-good compounds" and are released both to alleviate stress and pain and in response to enjoyable activities, such as laughing with friends over a particularly terrible Christmas cracker gag.

"It's not simply chuckling at a silly pun with a holiday cracker," she says. "You are in fact doing a lot of the really important task of making, maintaining the social bonds you have with those you care about."

What Occurs In the Brain?

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

An awful lot occurs in response to humour, it turns out.

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

Testing involves imaging the brains of volunteer participants and then exposing them to a database of humorous phrases, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we got a really fascinating activation pattern of neural activity," notes the neuroscientist.

A gag stimulates not just the areas of the brain responsible for auditory processing and interpreting speech, but also neural regions associated with both planning and starting movement and those linked to vision and memory.

Put these elements together, and individuals hearing a joke have a sophisticated series of neural reactions that support the laughter we hear.

The Infectious Power of Laughter

Scientists found that when a humorous phrase is combined with chuckles there is a stronger response in the mind than the same phrase when followed by a neutral sound.

"This was in parts of the brain that you would employ to contort your face into a smile or a chuckle," she says.

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

Amusement, according to the professor, can be infectious.

So what does this mean for the laughter found at a Christmas table?

"People laugh more when you know people," she says, "and laughter increases further when you are fond of them or care for them."

When it comes to Christmas cracker puns, she explains, the feel-good factor is more likely to be triggered not by the joke itself, but from the reaction to it.

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

The Quest for the Perfect Festive Pun

Is it possible to discover the perfect gag?

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

In 2001, a psychologist established a research search for the planet's most humorous joke.

More than tens of thousands of jokes submitted, with scores provided by 350,000 people around the world, he has a better idea than most as to what works and what fails.

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

"They must also need to be poor gags, puns that make us moan," he continues.

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

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

"What's interesting about the holiday cracker jokes is that not one person considers them funny.

"That's a shared moment around the gathering and I think it's lovely."