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

This joke is greeted with moans that resonate through a warehouse in London.

This describes a humor-evaluation meeting with a firm that produces products for social events. Its repertoire includes festive crackers.

The firm's founder grins, almost sheepishly at the joke. But the joke has made the cut and will appear in future crackers.

"The success is gauged by the joke by the number of groans and the intensity of the groans around the table," she explains.

The key to a good 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 Christmas meal with grandparents, children and possibly friends.

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

The Science Behind Shared Laughter

Gathering to enjoy communal laughter is not only nothing new, experts say, it is likely to be older than humanity.

"So when you are laughing with people at the Christmas table you are engaging in what's very likely a truly primordial mammal social vocalisation," says a neuroscience expert.

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

Scientists have found that a lack of such social exchanges can seriously damage mental and physical health.

"The people you converse with, and share laughter with, it results in increased amounts of 'happy chemical' release," the professor continues.

These natural chemicals are the body's "feel-good compounds" and are produced both to alleviate tension and discomfort and in response to pleasurable experiences, such as chuckling with friends over a particularly terrible festive cracker gag.

"You're not just laughing at a foolish pun with a holiday cracker," the expert states. "You are in fact doing a lot of the really vital work of building, preserving the social bonds you have with the people you love."

What Happens Inside the Mind?

But what is truly happening inside the mind when we hear a gag?

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

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

The research involves scanning the brains of healthy subjects and then exposing them to a collection of funny words, 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 stimulates not just the parts of the brain responsible for hearing and interpreting language, but also neural regions associated with both preparation and starting motion and those linked to vision and recall.

Combine all of this as a whole, and individuals hearing a joke have a complex set of brain reactions that support the laughter we experience.

The Contagious Nature of Laughter

Researchers discovered that when a funny word is paired with chuckles there is a stronger response in the brain than the same phrase when followed by a non-emotional sound.

"This activation occurred in areas of the brain that you would use to contort your face into a grin or a chuckle," she says.

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

Laughter, according to the professor, can be infectious.

So what does this mean for the laughter heard at a Christmas table?

"You laugh harder when you know others," she says, "and laughter increases further when you like them or care for them."

When it comes to festive cracker puns, she says, the feel-good factor is more probable to be caused not by the joke itself, but from the response to it.

"It's the laughter. The gag is the terrible Christmas cracker joke, and it's just a reason to laugh together."

The Search for the Perfect Festive Pun

Will we ever find the ultimate joke?

Likely not, but that has not prevented experts from trying to.

Years ago, a psychologist established a research project for the world's most humorous joke.

More than 40,000 jokes later, with ratings lodged by hundreds of thousands of participants around the world, he has a better understanding than many as to what works and what does not.

The perfect Christmas cracker pun needs to be brief, he explains.

"But they also need to be poor jokes, puns that cause us to moan," he continues.

The increasingly "awful" the gag, he states the more effective.

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

"What's interesting about the holiday cracker jokes is that none of us find them funny.

"That's a shared experience around the gathering and I believe it's wonderful."