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

This quip is met by groans that resonate through a warehouse in London.

This describes a humor-evaluation meeting with a firm that makes products for social events. Its catalogue features Christmas crackers.

The firm's founder smiles, almost apologetically at the joke. But the joke has made the cut and will appear in upcoming crackers.

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

The secret to a great Christmas cracker pun is not the same as a good joke per se. It is entirely about the context - in this instance, the communal laughter of the holiday dinner table with elders, children and potentially neighbours.

"You want the gag to be a thing that unites the eight-year-old together with the grandparent," she adds.

The Science Behind Communal Laughter

Gathering to experience shared amusement is not only nothing new, scientists argue, it is likely to be older than humanity.

"So when you are laughing with people around the Christmas dinner you are engaging in what's almost certainly a really primordial mammalian play vocalisation," explains a neuroscience expert.

Shared laughter, she explains, helps make and maintain social connections between individuals.

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

"Those you talk to, and share laughter with, it results in increased amounts of 'happy chemical' release," she continues.

Endorphins 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 loved ones over a particularly awful Christmas cracker joke.

"You're not just chuckling at a silly pun with a holiday cracker," the expert states. "You are in fact performing a lot of the truly important task of making, maintaining the connections you have with those you love."

Which Occurs Inside the Mind?

But what is truly happening inside the brain when we listen to a joke?

A tremendous amount occurs in response to humour, it turns out.

Using brain scanning technology, a type of neural imager which indicates which areas of the mind are working harder, researchers have been able to chart the areas that receive more blood flow.

The research involves scanning the brains of volunteer participants and then subjecting them to a database of funny phrases, accompanied by either a non-emotional sound, or pre-recorded laughter.

"During the study we got a very interesting activation pattern of neural activity," says the neuroscientist.

A joke activates not just the parts of the brain in charge of auditory processing and interpreting language, but also brain regions involved in both planning and initiating motion and those linked to vision and recall.

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

The Infectious Nature of Laughter

Scientists found that when a funny phrase is combined with chuckles there is a greater reaction in the mind than the same phrase when followed by a neutral sound.

"This activation occurred in parts of the mind that you would employ to contort your face into a grin or a laugh," the professor says.

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

Amusement, according to the expert, can be infectious.

So what does this imply for the chuckles found around a holiday gathering?

"You laugh harder when you know others," she says, "and laughter increases more when you are fond of them or care for them."

When it comes to festive cracker jokes, she says, the feel-good factor 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 Christmas cracker joke, and it's just a pretext to chuckle as a group."

The Search for the Ideal Festive Pun

Will we ever find the perfect joke?

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

In 2001, a professor established a research project for the world's funniest gag.

More than 40,000 gags later, with ratings lodged by 350,000 participants around the world, he has a clearer idea than many as to what works and what does not.

The ideal festive cracker pun needs to be brief, he explains.

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

The more "awful" the gag, he says the better.

"This is because if nobody finds it funny – it's the gag's shortcoming, not yours.

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

"It creates a common experience around the table and I believe it's wonderful."