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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 storage facility in London.

This describes a humor-evaluation meeting with a company that produces products for social events. Its catalogue features festive crackers.

The company's owner smiles, almost sheepishly at the joke. But the pun has made the cut and will feature in future crackers.

"You measure the gag 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 same as a good gag per se. It is entirely about the setting - in this case, the communal amusement of the holiday meal with grandparents, children and potentially neighbours.

"The goal is for the joke to be a thing that unites the eight-year-old together with the 80-year-old," she states.

The Neuroscience Of Communal Amusement

Gathering to enjoy shared laughter is not only ancient, scientists say, it is likely to be pre-human.

"So when you are chuckling with people at the holiday table you are engaging in what's very likely a really ancient mammal play sound," says a neuroscience expert.

Shared amusement, she says, helps forge and strengthen social bonds between people.

Scientists have found that a lack of these interactions can seriously damage mental and physical health.

"The people you talk to, and share laughter with, it leads to increased levels of endorphin uptake," she continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to enjoyable experiences, such as chuckling with friends over a particularly awful Christmas cracker gag.

"It's not simply chuckling at a silly joke with a Christmas cracker," she states. "You are in fact doing a lot of the really important work of building, preserving the social bonds you have with the people you care about."

Which Happens In the Mind?

But what is actually taking place within the mind when we listen to a joke?

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

Employing functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which parts of the mind are working harder, scientists have been able to chart the regions that get more blood flow.

Testing involves scanning the brains of volunteer 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 got a very fascinating activation pattern of activation," notes the neuroscientist.

A gag stimulates not just the parts of the mind in charge of auditory processing and understanding language, but also neural regions involved in both preparation and starting movement and those involved in vision and recall.

Put these elements as a whole, and individuals hearing a joke have a complex set of neural reactions that support the amusement we hear.

The Infectious Nature of Chuckles

Scientists discovered that when a funny word is paired with chuckles there is a stronger response in the mind than the identical word when followed by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to move your expression into a smile or a chuckle," the professor explains.

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

Laughter, according to the professor, can be infectious.

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

"People laugh more when you know people," she notes, "and laughter increases further when you like them or love them."

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

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

The Quest for the Ideal Festive Pun

Will we ever discover the perfect joke?

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

In 2001, a psychologist established a scientific search for the planet's funniest gag.

More than 40,000 gags later, with ratings provided by 350,000 people globally, he has a better understanding than most as to what succeeds and what does not.

The perfect festive cracker joke must be brief, he says.

"They must also be poor jokes, puns that make us moan," he continues.

The increasingly "awful" the gag, he states the better.

"This is because if no-one finds it funny – it's the joke's fault, not your own.

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

"That's a shared moment around the table and I think it's lovely."