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

This one-liner is met by moans that resonate through a storage facility in London.

We're at a humor-evaluation meeting with a firm that produces supplies for social events. Its repertoire features Christmas crackers.

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

"The success is gauged by the joke by the volume of moans and the intensity of the groans around the table," she says.

The secret to a great holiday cracker pun is not the identical as a stand-up gag in itself. It is all about the setting - in this case, the communal amusement of the holiday meal with elders, children and potentially friends.

"The goal is for the gag to be something that brings the child in harmony with the 80-year-old," she adds.

The Neuroscience Of Shared Amusement

Coming together to experience communal laughter is not only nothing new, experts argue, it is likely to be pre-human.

"So when you are laughing with others at the Christmas dinner you are dropping into what's almost certainly a truly primordial mammalian play vocalisation," says a professor.

Communal laughter, she explains, aids in make and maintain social connections between people.

Scientists have found that a lack of such interactions can significantly damage mental and physical health.

"The people you talk to, and laugh with, it leads to enhanced amounts of endorphin uptake," she adds.

Endorphins are the body's "happy chemicals" and are released both to reduce tension and discomfort and in response to enjoyable experiences, such as chuckling with friends over a truly terrible festive cracker joke.

"It's not simply laughing at a foolish joke with a holiday cracker," she says. "You are actually performing a lot of the really vital task of making, maintaining the social bonds you have with the people you love."

Which Happens In the Brain?

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

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which areas of the brain are working harder, researchers have been able to map the regions that receive more blood flow.

The research involves imaging the minds of volunteer subjects and then subjecting them to a database of humorous phrases, accompanied by either a non-emotional sound, or recorded laughter.

"During the study we observed a very interesting pattern of activation," says the professor.

A joke activates not just the parts of the brain in charge of auditory processing and interpreting speech, but also brain areas associated with both preparation and initiating motion and those involved in vision and recall.

Put these elements as a whole, and individuals hearing a pun have a sophisticated series of neural reactions that support the amusement we hear.

The Contagious Nature of Chuckles

Researchers found that when a humorous phrase is paired with laughter there is a stronger reaction in the mind than the identical phrase when accompanied 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 chuckle," the professor explains.

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

Laughter, according to the expert, can be infectious.

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

"People laugh more when you know people," she notes, "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 probable to be triggered not by the joke in itself, but from the response to it.

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

The Quest for the Perfect Festive Pun

Will we ever discover the ultimate gag?

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

Years ago, a psychologist set up a research project for the planet's most humorous gag.

More than 40,000 gags submitted, with ratings lodged by hundreds of thousands of people around the world, he has a better understanding than many as to what works and what fails.

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

"They must also be poor gags, puns that make us moan," he adds.

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

"The reason is that if no-one laughs – it's the gag's shortcoming, not your own.

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

"It creates a common experience at the table and I think it's lovely."