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

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

We're at a humor-evaluation session with a company that produces products for gatherings. Its catalogue includes Christmas crackers.

The firm's founder grins, nearly apologetically at the gag. But the joke has been selected and will appear in upcoming crackers.

"The success is gauged by the gag by the volume of moans and the intensity of the groans at the table," she says.

The key to a great Christmas cracker pun is not the same as a good gag in itself. It is all about the setting - in this instance, the communal laughter of the Christmas dinner table with elders, kids and possibly neighbours.

"You want the joke to be a thing that brings the eight-year-old in harmony with the 80-year-old," she states.

The Neuroscience Behind Shared Amusement

Coming together to experience shared laughter is not only ancient, experts say, it is probably to be older than humanity.

"So when you are chuckling with people around the holiday table you are dropping into what's very likely a really ancient mammal play vocalisation," says a neuroscience expert.

Shared amusement, she says, helps make and maintain social connections between individuals.

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

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

Endorphins are the brain's "happy chemicals" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as chuckling with loved ones over a truly terrible Christmas cracker joke.

"It's not simply chuckling at a foolish pun with a holiday cracker," the expert states. "You are in fact performing a lot of the truly important task of building, preserving the connections you have with the people you care about."

Which Occurs Inside the Mind?

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

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

Employing brain scanning technology, a kind of neural imager which indicates which parts of the mind are more active, scientists have been able to map the regions that get more blood.

The research entails scanning the brains of volunteer participants and then exposing them to a database of humorous phrases, paired with either a neutral sound, or pre-recorded laughter.

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

A gag stimulates not just the areas of the mind responsible for hearing and understanding language, but also brain regions involved in both planning and initiating motion and those linked to vision and memory.

Combine all of this together, and individuals listening to a joke have a complex series of neural responses that support the amusement we hear.

The Contagious Power of Laughter

Researchers found that when a funny word is combined with chuckles there is a stronger reaction in the brain than the identical phrase when followed by a non-emotional sound.

"This activation occurred in parts of the mind that you would employ to move your expression into a grin or a laugh," she explains.

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

Laughter, says the professor, can be contagious.

So what does this mean for the laughter heard at a Christmas gathering?

"People laugh more when you know others," she says, "and laughter increases further when you are fond of them or care for them."

When it comes to Christmas cracker jokes, she explains, the positive factor is more probable to be caused not by the gag 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 chuckle together."

The Quest for the Ideal Cracker Joke

Will we ever discover the ultimate joke?

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

In 2001, a psychologist established a research project for the planet's most humorous joke.

More than 40,000 jokes submitted, with scores provided by hundreds of thousands of participants globally, he has a better understanding than many as to what works and what fails.

The perfect festive cracker pun must be short, he says.

"They must also need to be poor gags, puns that cause us to moan," he adds.

The increasingly "awful" the gag, he says the more effective.

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

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

"That's a common moment at the gathering and I believe it's lovely."