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

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

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

The company's owner smiles, almost apologetically at the joke. But the pun has been selected and will appear in future crackers.

"You measure the joke by the volume of groans and the loudness of the groans around the table," the founder says.

The secret to a great Christmas cracker joke is not the identical as a good joke in itself. It is entirely about the context - in this instance, the shared amusement of the Christmas meal with grandparents, kids and potentially friends.

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

The Neuroscience Behind Shared Amusement

Coming together to experience shared laughter is not only ancient, scientists argue, it is likely to be pre-human.

"Therefore when you are laughing with others around the Christmas table you are engaging in what's very likely a truly primordial mammal play vocalisation," explains a professor.

Shared amusement, she explains, aids in make and maintain social connections between people.

Scientists have discovered that a lack of these interactions can seriously harm mental and physical well-being.

"The people you converse with, and laugh with, it leads to enhanced amounts of 'happy chemical' release," the professor continues.

Endorphins are the body's "feel-good compounds" and are released both to reduce tension and discomfort and in response to enjoyable experiences, such as chuckling with friends over a particularly terrible festive cracker gag.

"You're not just chuckling at a silly pun with a Christmas cracker," the expert says. "You are in fact doing a lot of the truly important task of making, maintaining the social bonds you have with the people you love."

What Happens In the Brain?

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

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

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 map the areas that get more blood flow.

The research entails imaging the brains of healthy participants and then subjecting them to a database of humorous phrases, accompanied by either a neutral sound, or pre-recorded laughter.

"In the scanner we got a really fascinating pattern of activation," says the neuroscientist.

A joke activates not just the areas of the mind in charge of auditory processing and understanding language, but also neural areas involved in both planning and initiating motion and those involved in vision and recall.

Combine all of this together, and people listening to a pun have a sophisticated set of neural responses that underpin the laughter we hear.

The Contagious Nature of Chuckles

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

"This was in parts of the brain that you would employ to contort your expression into a smile or a laugh," she says.

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

Amusement, says the expert, can be infectious.

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

"You laugh more when you know people," she says, "and laughter increases more when you like them or love them."

When it comes to Christmas cracker puns, she says, the feel-good effect is more likely 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 laugh as a group."

The Quest for the Ideal Cracker Joke

Is it possible to find the ultimate joke?

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

Years ago, a professor set up a scientific search for the world's most humorous joke.

More than tens of thousands of jokes later, with ratings provided by 350,000 people globally, he has a clearer understanding than many as to what works and what does not.

The ideal Christmas cracker joke must be brief, he says.

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

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

"This is because if no-one finds it funny – it's the joke's shortcoming, not yours.

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

"It creates a common moment at the gathering and I think it's lovely."