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

This joke is greeted with groans that echo through a storage facility in London.

We're at a joke-testing meeting with a company that produces supplies for social events. Its repertoire features Christmas crackers.

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

"The success is gauged by the gag by the number of groans and the intensity of the groans at the table," she says.

The key to a great Christmas cracker joke is not the identical as a stand-up gag in itself. It is entirely about the context - in this case, the communal laughter of the Christmas dinner table with elders, children and potentially friends.

"The goal is for the joke to be something that unites the child together with the 80-year-old," she adds.

The Science Of Communal Laughter

Gathering to enjoy shared laughter is not only nothing new, experts argue, it is probably to be older than humanity.

"Therefore when you are chuckling with people at the Christmas dinner you are engaging in what's very likely a really primordial mammal social vocalisation," says a professor.

Communal amusement, she says, helps make and maintain social connections between individuals.

Scientists have found that a absence of these interactions can significantly damage mental and physical health.

"The people you converse with, and laugh with, it results in enhanced levels of endorphin release," the professor adds.

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

"You're not just chuckling at a foolish pun with a holiday cracker," the expert says. "You are in fact performing a lot of the really important task of making, maintaining the social bonds you have with the people you care about."

What Happens Inside the Brain?

But what is truly taking place inside the mind when we listen to a gag?

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the mind are working harder, researchers have been able to chart the areas that receive more blood flow.

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

"In the scanner we got a very fascinating activation pattern of activation," notes the neuroscientist.

A gag stimulates not just the parts of the brain in charge of hearing and interpreting speech, but also neural regions associated with both planning and initiating motion and those linked to vision and recall.

Put these elements together, and people listening to a joke have a sophisticated series of brain reactions that underpin the amusement we hear.

The Infectious Power of Laughter

Scientists discovered that when a funny word is paired with chuckles there is a stronger reaction in the brain than the identical word when followed by a neutral sound.

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

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

Amusement, according to the expert, can be infectious.

So what does this imply for the chuckles heard at a Christmas gathering?

"People laugh harder when you know people," she says, "and you laugh further when you are fond of 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 joke in itself, but from the reaction to it.

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

The Quest for the Perfect Cracker Joke

Will we ever discover the ultimate gag?

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

Years ago, a psychologist set up a research project for the world's funniest joke.

More than tens of thousands of gags later, with scores lodged by hundreds of thousands of participants around the world, he has a better idea than most as to what succeeds and what does not.

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

"But they also be poor gags, jokes that cause us to moan," he adds.

The increasingly "terrible" the gag, he says the better.

"This is because if no-one laughs – it's the joke's shortcoming, not your own.

"The fascinating part about the holiday cracker puns is that none of us considers them funny.

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