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"How much did Santa's sled cost? Nothing, 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 session with a firm that produces supplies for gatherings. Its repertoire includes festive crackers.

The firm's owner smiles, almost sheepishly at the joke. But the pun has been selected and will feature in future crackers.

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

The secret to a good holiday cracker joke is not the identical as a good gag in itself. It is entirely about the setting - in this case, the shared amusement of the Christmas meal with elders, children and potentially friends.

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

The Science Behind Communal Amusement

Gathering to experience communal amusement is not only ancient, scientists say, it is probably to be pre-human.

"Therefore when you are laughing with others at the Christmas table you are dropping into what's almost certainly a truly primordial mammal social sound," explains a neuroscience expert.

Shared laughter, she explains, helps make and maintain social connections between individuals.

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

"The people you talk to, and share laughter with, it results in increased amounts of endorphin release," she adds.

Endorphins are the brain's "feel-good compounds" and are produced both to alleviate tension and discomfort and in reaction to enjoyable experiences, such as chuckling with loved ones over a particularly terrible Christmas cracker gag.

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

What Happens In the Mind?

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

A tremendous amount happens in response to comedy, it turns out.

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which parts of the mind are more active, scientists have been able to chart the areas that receive more blood.

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

"In the scanner we observed a really interesting pattern of activation," says the professor.

A joke activates not just the parts of the brain responsible for hearing and understanding language, but also brain regions associated with both preparation and starting movement and those linked to vision and recall.

Combine these elements together, and people listening to a joke have a complex set of neural responses that support the laughter we hear.

The Infectious Nature of Chuckles

Scientists discovered that when a humorous word is paired with laughter there is a greater reaction in the mind than the same phrase when accompanied by a non-emotional sound.

"This activation occurred in areas of the brain that you would use to move your face into a grin or a chuckle," the professor says.

It indicates people are not just responding to funny jokes, they are reacting to the amusement that accompanies them.

Amusement, according to the professor, can be infectious.

So what does this imply for the laughter heard around a holiday gathering?

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

"It's the laughter. The gag is the terrible Christmas cracker joke, and it's just a reason to chuckle together."

The Quest for the Ideal Cracker Joke

Will we ever discover the perfect gag?

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

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

More than 40,000 jokes later, with ratings lodged by hundreds of thousands of participants around the world, he has a clearer understanding than many as to what succeeds and what fails.

The perfect Christmas cracker joke must be brief, he explains.

"But they also be poor gags, jokes that make us groan," he adds.

The increasingly "terrible" the gag, he states the better.

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

"The fascinating part about the Christmas cracker puns is that none of us find them funny.

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