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"How much did Father Christmas's sleigh cost? Nothing, it was on the house."

This one-liner is met by moans that resonate through a storage facility in the capital.

We're at a joke-testing session with a company that makes products for social events. Its catalogue features festive crackers.

The company's owner grins, almost apologetically at the gag. But the pun has made the cut and will feature in future crackers.

"You measure the joke by the number of moans and the intensity of the groans at the table," she explains.

The secret to a great holiday cracker joke is not the same as a good joke in itself. It is all about the context - in this case, the shared amusement of the Christmas dinner table with grandparents, kids and potentially neighbours.

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

The Science Behind Shared Amusement

Gathering to enjoy communal laughter is not only nothing new, scientists say, it is probably to be pre-human.

"So when you are chuckling with others at the holiday dinner you are engaging in what's almost certainly a truly ancient mammal social vocalisation," explains a neuroscience expert.

Shared laughter, she explains, helps make and maintain social bonds between people.

Scientists have discovered that a absence of such social exchanges can significantly harm mental and physical health.

"The people you talk to, and share laughter with, it leads to enhanced levels of endorphin release," she adds.

These natural chemicals 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 truly awful Christmas cracker gag.

"You're not just chuckling at a silly joke with a holiday cracker," she states. "You are in fact doing a lot of the truly important work of making, maintaining the social bonds you have with the people you love."

Which Occurs Inside the Mind?

But what is actually taking place within the brain when we listen to a gag?

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

Using brain scanning technology, a kind of neural imager which indicates which parts of the mind are working harder, researchers have been able to map the regions that receive more blood flow.

The research involves scanning the minds of healthy subjects and then subjecting them to a database of funny words, accompanied by either a neutral sound, or recorded chuckles.

"During the study we observed a really interesting pattern of activation," says the neuroscientist.

A gag activates not just the parts of the mind in charge of hearing and interpreting speech, but also neural regions involved in both planning and starting movement and those involved in sight and recall.

Put these elements together, and individuals listening to a pun have a complex set of brain reactions that underpin the amusement we hear.

The Infectious Nature of Chuckles

Researchers discovered that when a humorous word is paired with laughter there is a greater response in the brain than the same phrase when followed by a neutral sound.

"This activation occurred in parts of the brain that you would use to move your expression into a smile or a chuckle," she says.

It means people are not just responding to funny words, they are responding to the laughter that accompanies them.

Laughter, says the professor, can be contagious.

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

"People laugh harder when you are familiar with people," she says, "and you laugh further when you like them or love them."

When it comes to Christmas cracker jokes, she explains, the positive effect is more probable to be triggered not by the gag in itself, but from the response to it.

"The laughter is key. The gag is the dreadful holiday cracker joke, and it's just a pretext to chuckle together."

The Search for the Perfect Festive Pun

Will we ever find the perfect gag?

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

Years ago, a professor established a research search for the planet's funniest joke.

More than tens of thousands of gags later, with ratings provided by hundreds of thousands of participants globally, he has a better understanding than many as to what works and what does not.

The ideal festive cracker pun must be brief, he explains.

"But they also need to be poor jokes, puns that cause us to moan," he adds.

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

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

"The fascinating part about the Christmas cracker puns is that not one person find them humorous.

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