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"What was the price did Santa's sled cost? Zero, it was on the house."

This one-liner is greeted with moans that echo through a storage facility in the capital.

This describes a humor-evaluation meeting with a firm that produces products for gatherings. Its catalogue includes Christmas crackers.

The firm's founder smiles, nearly sheepishly at the gag. But the pun has been selected and will feature in future crackers.

"You measure the joke by the number of groans and the intensity of the groans at the table," she explains.

The key to a good Christmas cracker pun is not the same as a stand-up gag per se. It is entirely about the setting - in this case, the communal amusement of the Christmas dinner table with grandparents, kids and potentially friends.

"The goal is for the joke to be something that brings the eight-year-old together with the grandparent," she adds.

The Neuroscience Of Communal Amusement

Coming together to experience communal amusement is not only ancient, scientists say, it is probably to be pre-human.

"So when you are chuckling with people at the holiday dinner you are dropping into what's very likely a really ancient mammal play vocalisation," says a professor.

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

Researchers have found that a lack of these social exchanges can seriously harm mental and physical health.

"Those you talk to, and laugh with, it results in increased amounts of 'happy chemical' uptake," the professor adds.

Endorphins are the body's "happy chemicals" and are released both to alleviate tension and discomfort and in response to pleasurable experiences, such as laughing with friends over a particularly awful Christmas cracker gag.

"It's not simply chuckling at a silly joke with a Christmas cracker," the expert says. "You are in fact doing a lot of the really vital task of making, maintaining the connections you have with those you care about."

What Happens In the Mind?

But what is truly taking place within the mind when we listen to a gag?

A tremendous amount happens in reaction to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which parts of the brain are working harder, researchers have been able to map the regions that get more blood.

Testing entails scanning the brains of healthy participants and then exposing them to a database of humorous phrases, accompanied by either a neutral sound, or pre-recorded laughter.

"During the study we observed a very interesting activation pattern of activation," notes the neuroscientist.

A joke activates not just the parts of the mind responsible for hearing and interpreting speech, but also brain regions involved in both preparation and starting movement and those involved in sight and recall.

Put these elements together, and individuals listening to a joke have a complex series of neural responses that support the laughter we experience.

The Infectious Power of Chuckles

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

"This activation occurred in areas of the mind that you would use to contort your expression into a grin or a chuckle," the professor says.

It means we are not just reacting to humorous jokes, they are reacting to the amusement that accompanies them.

Amusement, according to the expert, can be contagious.

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

"You laugh harder when you know people," she says, "and you laugh more when you like them or care for them."

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

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the ultimate joke?

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

In 2001, a professor set up a research project for the planet's funniest joke.

More than 40,000 gags submitted, with scores provided by hundreds of thousands of participants around the world, he has a better understanding than most as to what works and what fails.

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

"But they also need to be bad gags, puns that cause us to groan," he adds.

The more "awful" the joke, he says the more effective.

"This is because if nobody 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 find them funny.

"That's a shared experience at the table and I believe it's wonderful."