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

This one-liner is met by moans that echo through a warehouse in the capital.

We're at a joke-testing session with a company that produces supplies for gatherings. Its catalogue features festive crackers.

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

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

The secret to a good Christmas cracker joke is not the same as a stand-up gag per se. It is entirely about the setting - in this instance, the shared laughter of the holiday dinner table with elders, kids and possibly neighbours.

"You want the gag to be something that unites the eight-year-old together with the 80-year-old," she adds.

The Science Of Communal Amusement

Gathering to enjoy shared amusement is not only ancient, experts say, it is probably to be older than humanity.

"So when you are laughing with people at the Christmas dinner you are dropping into what's almost certainly a really primordial mammalian play vocalisation," says a neuroscience expert.

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

Scientists have found that a absence of these social exchanges can significantly harm mental and physical well-being.

"Those you talk to, and laugh with, it results in increased amounts of endorphin release," she adds.

Endorphins are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in reaction to enjoyable experiences, such as laughing with loved ones over a truly awful Christmas cracker gag.

"It's not simply chuckling at a foolish joke with a holiday cracker," the expert says. "You are in fact doing a lot of the truly vital task of making, maintaining the social bonds you have with the people you care about."

What Happens Inside the Mind?

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

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

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which parts of the mind are working harder, scientists have been able to chart the regions that receive more blood flow.

Testing involves scanning the brains of volunteer 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 neuroscientist.

A gag activates not just the areas of the brain in charge of auditory processing and interpreting speech, but also neural regions associated with both preparation and initiating motion and those linked to sight and memory.

Combine all of this together, and people listening to a pun have a sophisticated series of brain responses that support the amusement we experience.

The Contagious Power of Chuckles

Researchers discovered that when a humorous word is combined with chuckles there is a stronger response in the brain than the same word when accompanied by a neutral sound.

"This was in areas of the mind that you would employ to contort your face into a smile 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.

Laughter, says the expert, can be infectious.

So what does this mean for the chuckles found around a Christmas gathering?

"You laugh harder when you know others," she notes, "and you laugh more when you like them or love them."

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

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

The Quest for the Perfect Festive Pun

Is it possible to discover the perfect gag?

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

In 2001, a professor established a scientific search for the world's most humorous gag.

Over 40,000 gags submitted, with scores provided by 350,000 participants globally, he has a clearer understanding than most as to what works and what does not.

The ideal Christmas cracker pun must be short, he explains.

"But they also be poor jokes, puns that make us moan," he adds.

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

"The reason is that if no-one finds it funny – it's the gag's shortcoming, not yours.

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

"That's a common moment at the table and I think it's lovely."