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

This one-liner is met by groans that echo through a storage facility in London.

We're at a humor-evaluation meeting with a company that makes products for social events. Its catalogue features festive crackers.

The company's owner smiles, almost sheepishly at the gag. But the joke has made the cut and will feature in future crackers.

"You measure the gag by the volume of groans and the intensity of the groans around the table," the founder explains.

The key to a good holiday cracker joke is not the same as a stand-up joke in itself. It is all about the context - in this instance, the shared laughter of the holiday meal with grandparents, children and potentially friends.

"The goal is for the gag to be something that unites the eight-year-old in harmony with the 80-year-old," she adds.

The Science Of Communal Laughter

Coming together to enjoy communal laughter is not only nothing new, experts say, it is probably to be older than humanity.

"So when you are laughing with others at the Christmas dinner you are engaging in what's very likely a really ancient mammalian play sound," says a neuroscience expert.

Communal amusement, she says, helps make and maintain social connections between people.

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

"Those you converse with, and laugh with, it leads to increased amounts of 'happy chemical' release," she adds.

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

"You're not just laughing at a foolish joke with a holiday cracker," the expert says. "You are in fact performing a lot of the truly important work of building, preserving the connections you have with the people you care about."

What Happens In the Brain?

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

A tremendous amount occurs in reaction to comedy, it turns out.

Employing brain scanning technology, a type of neural imager which indicates which parts of the mind are more active, scientists have been able to chart the areas that get more blood flow.

Testing entails imaging the brains of healthy participants and then subjecting them to a collection of funny phrases, accompanied by either a non-emotional sound, or pre-recorded laughter.

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

A gag activates not just the areas of the mind in charge of hearing and understanding language, but also brain regions involved in both planning and starting movement and those linked to sight and recall.

Put these elements as a whole, and individuals hearing a pun have a complex series of neural reactions that underpin the amusement we experience.

The Contagious Nature of Laughter

Scientists found that when a humorous phrase is paired with chuckles there is a greater reaction in the mind than the identical phrase when followed by a non-emotional sound.

"This activation occurred 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 reacting to humorous jokes, they are responding to the laughter that accompanies them.

Laughter, according to the professor, can be infectious.

So what does this mean for the chuckles found around a holiday gathering?

"People laugh more when you know others," she says, "and laughter increases more when you like them or love them."

When it comes to Christmas cracker jokes, she explains, the feel-good factor is more likely to be caused not by the gag itself, but from the response to it.

"It's the laughter. The gag is the terrible holiday cracker pun, and it's just a pretext to chuckle as a group."

The Search for the Perfect Festive Pun

Will we ever find the perfect gag?

Likely not, but that has not prevented experts from trying to.

In 2001, a psychologist set up a scientific project for the world's most humorous gag.

Over 40,000 gags later, with ratings lodged by hundreds of thousands of people around the world, he has a clearer idea than many as to what succeeds and what fails.

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

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

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

"The reason is that if nobody laughs – it's the joke's shortcoming, not your own.

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

"It creates a shared experience at the table and I believe it's wonderful."