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

This joke is met by moans that echo through a storage facility in London.

We're at a humor-evaluation meeting with a company that makes products for gatherings. Its catalogue includes festive crackers.

The company's owner grins, nearly apologetically at the gag. But the pun has been selected and will appear in upcoming crackers.

"The success is gauged by the joke by the volume of moans and the loudness of the groans at the table," the founder explains.

The key to a good holiday cracker joke is not the identical as a stand-up gag per se. It is entirely about the context - in this case, the shared laughter of the holiday meal with elders, children and potentially neighbours.

"You want the gag to be something that unites the child in harmony with the 80-year-old," she states.

The Neuroscience Of Shared Amusement

Gathering to enjoy shared amusement is not only ancient, scientists say, it is likely to be pre-human.

"So when you are chuckling with people around the Christmas table you are dropping into what's almost certainly a really primordial mammalian social sound," explains a professor.

Communal laughter, she says, aids in make and maintain social bonds between individuals.

Researchers have found that a lack of such social exchanges can seriously damage mental and physical well-being.

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

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce tension and discomfort and in reaction to enjoyable activities, such as chuckling with friends over a truly awful festive cracker joke.

"You're not just chuckling at a foolish joke with a Christmas cracker," she says. "You are actually doing a lot of the truly vital work of making, maintaining the connections you have with the people you love."

Which Occurs In the Brain?

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

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

Employing brain scanning technology, a type of brain scanner which indicates which parts of the mind are more active, researchers have been able to map the areas that receive more blood.

The research entails scanning the minds of volunteer participants and then subjecting them to a collection of humorous words, paired with either a non-emotional sound, or pre-recorded laughter.

"During the study we got a really fascinating activation pattern of neural activity," notes the neuroscientist.

A joke activates not just the areas of the mind responsible for auditory processing and interpreting speech, but also neural regions associated with both preparation and starting motion and those linked to vision and memory.

Put these elements together, and individuals listening to a joke have a sophisticated series of brain responses that underpin the laughter we hear.

The Infectious Nature of Chuckles

Researchers found that when a humorous word is combined with laughter there is a stronger response in the mind than the identical word when accompanied by a non-emotional sound.

"This was in areas of the mind that you would use to contort your face into a smile or a chuckle," the professor says.

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

Laughter, says the professor, can be contagious.

So what does this imply for the chuckles heard around a Christmas table?

"People laugh more when you are familiar with others," she says, "and laughter increases further when you are fond of them or love them."

When it comes to festive cracker puns, she says, the feel-good effect is more likely to be triggered not by the joke itself, but from the reaction to it.

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the ultimate joke?

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

Years ago, a professor set up a scientific project for the planet's funniest joke.

Over tens of thousands of gags later, with ratings lodged by 350,000 people around the world, he has a better understanding than many as to what succeeds and what does not.

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

"But they also be poor gags, puns that make us groan," he continues.

The more "awful" the joke, he states the better.

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

"The fascinating part about the Christmas cracker jokes is that none of us considers them humorous.

"It creates a common moment at the table and I believe it's lovely."