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

This one-liner is greeted with groans that resonate through a storage facility in London.

We're at a joke-testing session with a firm that makes supplies for social events. Its repertoire includes Christmas crackers.

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

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

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 case, the communal amusement of the holiday dinner table with grandparents, children and potentially neighbours.

"The goal is for the joke to be a thing that unites the child in harmony with the grandparent," she adds.

The Neuroscience Behind Shared Amusement

Coming together to enjoy shared amusement is not only nothing new, scientists argue, it is likely to be older than humanity.

"So when you are chuckling with people at the Christmas dinner you are engaging in what's very likely a really ancient mammalian social sound," says a professor.

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

Scientists have discovered that a lack of these interactions can significantly harm both psychological and bodily health.

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

These natural chemicals are the body's "happy chemicals" and are released both to reduce tension and discomfort and in response to enjoyable activities, such as chuckling with friends over a truly awful Christmas cracker joke.

"It's not simply laughing at a foolish pun with a Christmas cracker," she states. "You are in fact performing a lot of the really vital task of making, maintaining the connections you have with those you care about."

Which Happens Inside the Brain?

But what is truly taking place within the brain when we listen to a joke?

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

Using brain scanning technology, a kind of brain scanner which indicates which areas of the mind are working harder, scientists have been able to map the regions that receive more blood.

The research entails scanning the minds of healthy participants and then subjecting them to a database of funny phrases, accompanied by either a non-emotional sound, or pre-recorded chuckles.

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

A joke stimulates not just the areas of the brain responsible for auditory processing and understanding speech, but also brain regions associated with both preparation and starting motion and those involved in vision and memory.

Put these elements together, and people listening to a joke have a sophisticated set of brain responses that support the amusement we hear.

The Infectious Power of Laughter

Researchers found that when a funny phrase is combined with laughter there is a stronger response in the mind than the same word when followed by a non-emotional sound.

"This was in parts of the mind that you would employ to contort your face into a grin or a chuckle," she explains.

It indicates we are not just responding to funny jokes, they are responding to the laughter that accompanies them.

Laughter, according to the expert, can be infectious.

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

"People laugh more when you know people," she notes, "and laughter increases more when you are fond of them or love them."

When it comes to festive cracker jokes, she explains, the feel-good factor is more likely to be caused not by the gag in itself, but from the response to it.

"It's the laughter. The joke is the dreadful holiday cracker pun, and it's just a pretext to laugh together."

The Quest for the Perfect Cracker Joke

Is it possible to discover the perfect gag?

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

In 2001, a professor established a scientific project for the planet's most humorous gag.

Over tens of thousands of jokes submitted, with ratings provided by 350,000 people around the world, he has a better idea than many as to what works and what fails.

The ideal festive cracker joke needs to be brief, he explains.

"But they also need to be poor jokes, jokes that make us groan," he continues.

The more "terrible" the gag, he says the better.

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

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

"It creates a common moment around the table and I believe it's wonderful."