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

This quip is greeted with groans that resonate through a storage facility in the capital.

We're at a joke-testing meeting with a firm that makes products for gatherings. Its repertoire features festive crackers.

The company's founder grins, almost sheepishly at the joke. But the pun has been selected and will feature in future crackers.

"The success is gauged by the joke by the number of groans and the loudness of the groans around the table," the founder says.

The secret to a good holiday cracker pun is not the same as a stand-up joke in itself. It is all about the setting - in this instance, the communal amusement of the Christmas meal with grandparents, kids and potentially neighbours.

"You want the joke to be something that unites the eight-year-old in harmony with the 80-year-old," she adds.

The Neuroscience Behind Shared Amusement

Coming together to enjoy shared amusement is not only nothing new, scientists say, it is probably to be pre-human.

"So when you are chuckling with people around the Christmas dinner you are engaging in what's very likely a really primordial mammalian social vocalisation," explains a professor.

Communal laughter, she says, aids in forge and strengthen social bonds between individuals.

Scientists have found that a lack of these interactions can seriously damage mental and physical well-being.

"The people you talk to, and share laughter with, it leads to enhanced amounts of endorphin release," the professor adds.

Endorphins are the brain's "happy chemicals" and are produced both to alleviate tension and discomfort and in response to pleasurable activities, such as chuckling with loved ones over a truly awful festive cracker joke.

"It's not simply laughing at a silly pun with a Christmas cracker," the expert states. "You are in fact doing a lot of the truly important task of building, preserving the connections you have with the people you love."

What Happens Inside the Mind?

But what is truly happening within the brain when we hear a gag?

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

Employing brain scanning technology, a type of brain scanner which indicates which areas of the brain are working harder, scientists have been able to chart the regions that get more blood.

Testing entails scanning the brains of healthy participants and then exposing them to a collection of humorous words, accompanied by either a non-emotional sound, or pre-recorded laughter.

"In the scanner we observed a very fascinating activation pattern of activation," says the neuroscientist.

A gag activates not just the areas of the mind in charge of hearing and interpreting language, but also neural regions involved in both planning and initiating motion and those linked to sight and recall.

Combine all of this as a whole, and individuals hearing a pun have a complex set of brain responses that underpin the laughter we experience.

The Infectious Power of Laughter

Scientists found that when a humorous word is combined with laughter there is a greater response in the brain than the same phrase when accompanied by a neutral sound.

"This activation occurred in areas of the brain that you would employ to move your expression into a grin or a laugh," she says.

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

Laughter, according to the professor, can be contagious.

So what does this mean for the laughter found around a Christmas gathering?

"People laugh harder when you are familiar with others," she says, "and you laugh more when you are fond of them or care for them."

When it comes to Christmas cracker puns, she explains, the positive effect is more probable 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 reason to chuckle as a group."

The Quest for the Perfect Cracker Joke

Will we ever find the perfect gag?

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

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

More than 40,000 gags later, with scores provided by hundreds of thousands of people globally, he has a better idea than many as to what works and what does not.

The ideal Christmas cracker joke needs to be short, he explains.

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

The more "awful" the gag, he says the better.

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

"The fascinating part about the holiday cracker jokes is that not one person find them funny.

"That's a common moment around the gathering and I believe it's wonderful."