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

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

This describes a joke-testing session with a firm that produces products for social events. Its catalogue features Christmas crackers.

The firm's owner smiles, almost apologetically at the gag. But the joke has made the cut and will feature in future crackers.

"You measure the joke by the volume of groans and the loudness of the groans at the table," she explains.

The secret to a good holiday cracker joke is not the same as a good gag in itself. It is entirely about the context - in this instance, the communal laughter of the Christmas meal with grandparents, kids and possibly friends.

"You want the gag to be something that brings the eight-year-old in harmony with the 80-year-old," she states.

The Neuroscience Of Communal Laughter

Coming together to experience shared amusement is not only nothing new, scientists argue, it is probably to be pre-human.

"Therefore when you are laughing with others around the holiday dinner you are dropping into what's almost certainly a really ancient mammalian play vocalisation," says a neuroscience expert.

Communal laughter, she explains, aids in make and maintain social bonds between individuals.

Scientists have discovered that a lack of such social exchanges can seriously damage both psychological and bodily well-being.

"The people you converse with, and share laughter with, it leads to enhanced amounts of endorphin release," she adds.

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

"It's not simply laughing at a foolish pun with a holiday cracker," she says. "You are actually doing a lot of the truly vital work of building, preserving the social bonds you have with those you care about."

What Occurs In the Mind?

But what is truly happening within the brain when we listen to a gag?

An awful lot occurs in reaction to comedy, it turns out.

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

Testing involves scanning the minds of healthy participants and then exposing them to a collection of humorous phrases, paired with either a non-emotional sound, or pre-recorded laughter.

"In the scanner we observed a very interesting activation pattern of neural activity," notes the neuroscientist.

A gag activates not just the areas of the brain in charge of auditory processing and understanding speech, but also brain areas associated with both preparation and starting movement and those linked to vision and recall.

Put all of this as a whole, and individuals hearing a joke have a sophisticated set of neural responses that underpin the laughter we hear.

The Contagious Power of Laughter

Scientists found that when a funny word is paired with chuckles there is a greater reaction in the mind than the identical phrase when accompanied by a neutral sound.

"This activation occurred in areas of the mind that you would use to move your face into a smile or a laugh," she says.

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

Laughter, according to the expert, can be infectious.

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

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

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

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

The Search for the Ideal Festive Pun

Will we ever discover the perfect joke?

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

In 2001, a professor set up a research project for the planet's most humorous joke.

More than 40,000 jokes submitted, with scores provided by hundreds of thousands of participants globally, he has a better idea than many as to what works and what fails.

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

"But they also be bad gags, jokes that cause us to groan," he adds.

The increasingly "terrible" the joke, he states the better.

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

"What's interesting about the Christmas cracker puns is that not one person considers them humorous.

"It creates a shared moment at the gathering and I believe it's wonderful."