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"What was the price did Santa's sleigh cost? Nothing, it was on the house."

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

This describes a joke-testing session with a firm that produces products for social events. Its repertoire features festive crackers.

The firm's owner grins, almost sheepishly at the gag. But the joke has been selected and will appear in upcoming crackers.

"You measure the gag by the number of groans and the loudness of the groans around the table," she says.

The secret to a great Christmas cracker pun is not the identical as a good gag per se. It is entirely about the setting - in this instance, the communal amusement of the Christmas dinner table with grandparents, children and potentially friends.

"The goal is for the joke to be a thing that unites the child together with the 80-year-old," she states.

The Neuroscience Behind Shared Amusement

Gathering to enjoy communal amusement is not only nothing new, experts argue, it is probably to be older than humanity.

"Therefore when you are laughing with people at the holiday table you are engaging in what's almost certainly a truly ancient mammal social vocalisation," says a neuroscience expert.

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

Scientists have found that a absence of these social exchanges can seriously damage mental and physical well-being.

"The people you talk to, and share laughter with, it leads to increased amounts of 'happy chemical' uptake," the professor continues.

These natural chemicals are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in reaction to pleasurable activities, such as chuckling with loved ones over a truly terrible Christmas cracker joke.

"It's not simply laughing at a silly joke with a Christmas cracker," she says. "You are actually doing a lot of the really important work of building, preserving the social bonds you have with the people you care about."

Which Occurs Inside the Brain?

But what is actually happening inside the brain when we listen to a joke?

A tremendous amount happens in reaction to humour, it transpires.

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the mind are more active, researchers have been able to chart the regions that get more blood flow.

Testing entails scanning the brains of healthy subjects and then subjecting them to a collection of funny phrases, paired with either a neutral sound, or recorded chuckles.

"During the study we observed a very fascinating pattern of activation," notes the professor.

A gag activates not just the parts of the mind responsible for auditory processing and interpreting language, but also brain regions involved in both preparation and starting motion and those involved in vision and recall.

Put these elements as a whole, and people listening to a pun have a complex series of brain responses that support the amusement we hear.

The Contagious Nature of Laughter

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

"This was in parts of the brain that you would use to move your face into a smile or a laugh," she says.

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

Amusement, says the professor, can be infectious.

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

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

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

"The laughter is key. The joke is the terrible holiday cracker pun, and it's just a reason to chuckle together."

The Search for the Ideal Festive Pun

Is it possible to discover the perfect gag?

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

Years ago, a psychologist established a research search for the world's most humorous joke.

Over 40,000 jokes later, with scores lodged by hundreds of thousands of people globally, he has a clearer understanding than most as to what works and what does not.

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

"But they also be bad jokes, jokes that make us moan," he continues.

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

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

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

"It creates a common moment at the gathering and I believe it's wonderful."