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

This one-liner is met by groans that resonate through a storage facility in London.

This describes a joke-testing session with a company that makes supplies for social events. Its repertoire features Christmas crackers.

The firm's owner grins, nearly sheepishly at the joke. But the joke has been selected and will feature in future crackers.

"The success is gauged by the gag by the volume of groans and the loudness of the groans at the table," the founder explains.

The secret to a great holiday cracker joke is not the same as a good joke per se. It is entirely about the setting - in this instance, the communal amusement of the Christmas meal with grandparents, kids and potentially friends.

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

The Science Behind Communal Amusement

Gathering to enjoy communal amusement is not only ancient, scientists say, 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 almost certainly a truly primordial mammal play sound," says a neuroscience expert.

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

Researchers have discovered that a absence of such social exchanges can significantly harm mental and physical health.

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

These natural chemicals are the brain's "feel-good compounds" and are produced both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as laughing with friends over a truly awful Christmas cracker joke.

"You're not just chuckling at a silly pun with a Christmas cracker," the expert states. "You are in fact performing a lot of the really vital task of building, preserving the social bonds you have with the people you love."

What Happens In the Mind?

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

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

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which shows which areas of the mind are working harder, researchers have been able to map the areas that receive more blood flow.

The research involves imaging the minds of volunteer participants and then subjecting them to a database of funny words, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we got a very fascinating pattern of activation," says the professor.

A joke stimulates not just the areas of the mind in charge of hearing and understanding speech, but also brain regions involved in both preparation and initiating motion and those involved in vision and recall.

Put these elements together, and people listening to a pun have a sophisticated set of neural responses that underpin the amusement we experience.

The Infectious Nature of Laughter

Researchers found that when a humorous phrase is paired with laughter there is a greater reaction in the mind than the same phrase when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to contort your expression into a grin or a laugh," she says.

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

Amusement, says the professor, can be contagious.

So what does this mean for the chuckles found around a Christmas table?

"People laugh more when you know others," she says, "and you laugh further when you are fond of them or love them."

When it comes to Christmas cracker jokes, she explains, the feel-good effect is more likely to be caused not by the gag in itself, but from the response to it.

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

The Quest for the Ideal Cracker Joke

Will we ever find the perfect gag?

Likely not, but that has not stopped experts from trying to.

In 2001, a psychologist set up a scientific project for the planet's funniest gag.

Over 40,000 jokes submitted, with ratings lodged by 350,000 people globally, he has a clearer understanding than most as to what succeeds and what fails.

The ideal Christmas cracker joke needs to be brief, he explains.

"But they also be bad gags, puns that cause us to groan," 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 shortcoming, not yours.

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

"It creates a shared experience at the table and I think it's lovely."