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"How much did Santa's sleigh cost? Nothing, it was on the house."

This one-liner is met by groans that echo through a storage facility in the capital.

We're at a humor-evaluation session with a company that makes supplies for social events. Its repertoire features Christmas crackers.

The company's owner grins, almost sheepishly at the gag. But the joke has been selected and will feature in upcoming crackers.

"You measure the joke by the volume of moans and the intensity of the groans around the table," she explains.

The key to a great holiday cracker pun is not the identical as a good joke per se. It is all about the context - in this case, the shared amusement of the holiday dinner table with grandparents, children and potentially friends.

"You want the joke to be a thing that brings the child in harmony with the 80-year-old," she adds.

The Neuroscience Of Shared Laughter

Gathering to experience communal amusement is not only ancient, scientists say, it is likely to be older than humanity.

"So when you are laughing with others at the holiday dinner you are engaging in what's very likely a truly primordial mammal play sound," explains a neuroscience expert.

Shared laughter, she says, aids in forge and strengthen social connections between people.

Researchers have found that a lack of such social exchanges can significantly damage both psychological and bodily well-being.

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

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce tension and discomfort and in response to pleasurable activities, such as chuckling with loved ones over a particularly terrible Christmas cracker gag.

"You're not just laughing at a silly joke with a Christmas cracker," the expert says. "You are in fact performing a lot of the really important work of building, preserving the social bonds you have with the people you care about."

What Occurs Inside the Brain?

But what is actually happening within the mind when we hear a gag?

An awful lot happens in response to comedy, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which areas of the brain are more active, researchers have been able to chart the areas that get more blood.

The research entails imaging the brains of healthy subjects and then exposing them to a collection of funny words, accompanied by either a neutral sound, or recorded laughter.

"In the scanner we got a very interesting pattern of activation," says the neuroscientist.

A joke stimulates not just the areas of the mind in charge of hearing and understanding speech, but also brain regions associated with both planning and initiating motion and those linked to vision and memory.

Combine these elements together, and people hearing a joke have a sophisticated series of brain responses that support the laughter we experience.

The Infectious Power of Chuckles

Researchers discovered that when a funny phrase is combined with laughter there is a greater response in the brain than the same phrase when accompanied by a neutral sound.

"This was in areas of the brain that you would use to contort your face into a grin or a chuckle," the professor explains.

It indicates people are not just responding to funny words, they are responding to the laughter that follows them.

Amusement, according to the professor, can be infectious.

So what does this mean for the laughter found around a Christmas table?

"You laugh harder when you are familiar with people," she says, "and you laugh further when you are fond of them or care for them."

When it comes to Christmas cracker puns, she says, the positive effect is more likely to be caused not by the joke itself, but from the response to it.

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

The Quest for the Ideal Cracker Joke

Will we ever discover the perfect gag?

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

In 2001, a psychologist set up a scientific project for the planet's most humorous gag.

Over 40,000 gags submitted, with ratings provided by 350,000 people around the world, he has a clearer idea than most as to what succeeds and what fails.

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

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

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

"This is because if no-one laughs – it's the gag's shortcoming, not yours.

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

"That's a shared experience around the table and I believe it's lovely."