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

This joke is greeted with moans that resonate through a storage facility in London.

This describes a joke-testing session with a firm that produces products for gatherings. Its repertoire features Christmas crackers.

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

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

The key to a good Christmas cracker pun is not the identical 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 elders, children and possibly friends.

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

The Science Of Communal Amusement

Gathering to experience shared amusement is not only ancient, scientists say, it is probably to be pre-human.

"Therefore when you are laughing with others at the holiday table you are engaging in what's almost certainly a truly primordial mammal play sound," explains a neuroscience expert.

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

Scientists have discovered that a lack of such social exchanges can seriously damage mental and physical health.

"Those you talk to, and share laughter with, it results in increased amounts of endorphin uptake," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to pleasurable activities, such as chuckling with loved ones over a truly terrible festive cracker gag.

"You're not just chuckling at a silly joke with a Christmas cracker," the expert says. "You are actually doing a lot of the really vital task of building, preserving the connections you have with those you care about."

Which Occurs Inside the Brain?

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

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

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

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

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

A gag stimulates not just the parts of the brain responsible for auditory processing and understanding speech, but also brain areas associated with both planning and initiating motion and those linked to sight and recall.

Combine these elements as a whole, and individuals hearing a joke have a complex series of neural responses that support the laughter we experience.

The Infectious Power of Chuckles

Scientists discovered that when a funny phrase is combined with chuckles there is a stronger response in the mind than the identical word when accompanied by a neutral sound.

"This activation occurred in parts of the brain that you would use to move your face into a grin or a chuckle," the professor says.

It means people are not just responding to humorous words, they are reacting to the laughter that follows them.

Amusement, says the expert, can be contagious.

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

"People laugh harder when you know people," she says, "and you laugh more when you like them or care for them."

When it comes to Christmas cracker puns, she says, the positive factor is more likely to be caused not by the gag itself, but from the reaction to it.

"It's the laughter. The gag is the terrible Christmas cracker pun, and it's just a reason to chuckle together."

The Quest for the Ideal Cracker Joke

Will we ever discover the perfect gag?

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

In 2001, a psychologist established a scientific project for the world's most humorous gag.

Over 40,000 gags submitted, with scores lodged by 350,000 people globally, he has a clearer idea than most as to what succeeds and what does not.

The ideal festive cracker pun must be short, he says.

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

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

"This is because if nobody finds it funny – it's the gag's fault, not your own.

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

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