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"How much did Father Christmas's sled cost? Zero, it was on the house."

This quip is greeted with moans that resonate through a storage facility in London.

This describes a joke-testing session with a firm that makes products for social events. Its repertoire includes Christmas crackers.

The firm's founder smiles, almost apologetically at the gag. But the pun has been selected and will appear in upcoming crackers.

"You measure the gag by the number of moans and the loudness of the groans at the table," the founder explains.

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

"You want the gag to be something that unites the eight-year-old together with the grandparent," she adds.

The Neuroscience Of Communal Laughter

Gathering to enjoy communal laughter is not only nothing new, scientists say, it is likely to be older than humanity.

"Therefore when you are laughing with people at the Christmas table you are engaging in what's very likely a really ancient mammal social vocalisation," explains a neuroscience expert.

Shared amusement, she says, aids in make and maintain social connections between individuals.

Researchers have found that a lack of these interactions can seriously damage mental and physical health.

"Those you talk to, and laugh with, it results in increased amounts of endorphin uptake," she continues.

Endorphins are the body's "feel-good compounds" and are released both to alleviate tension and discomfort and in response to enjoyable activities, such as chuckling with loved ones over a truly awful festive cracker gag.

"It's not simply chuckling at a foolish pun with a holiday cracker," she states. "You are in fact doing a lot of the really vital work of making, maintaining the connections you have with the people you care about."

What Occurs In the Brain?

But what is truly taking place within the mind when we listen to a joke?

A tremendous amount happens in response to comedy, it transpires.

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

The research involves imaging the brains of healthy subjects and then exposing them to a database of humorous words, paired with either a non-emotional sound, or pre-recorded chuckles.

"During the study we observed a really fascinating pattern of neural activity," says the professor.

A joke activates not just the areas of the brain in charge of hearing and interpreting language, but also brain areas involved in both planning and initiating motion and those involved in vision and recall.

Put all of this as a whole, and individuals hearing a joke have a complex series of brain responses that support the laughter we experience.

The Contagious Power of Chuckles

Researchers discovered that when a humorous word is paired with laughter there is a stronger response in the mind than the identical word when followed by a non-emotional sound.

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

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

Laughter, according to the expert, can be contagious.

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

"People laugh harder when you know people," she notes, "and laughter increases more when you like them or love them."

When it comes to festive cracker puns, she explains, the positive factor is more probable to be caused not by the joke in itself, but from the response to it.

"It's the laughter. The gag is the dreadful holiday cracker joke, and it's just a reason to chuckle as a group."

The Quest for the Ideal Festive Pun

Will we ever find the perfect joke?

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

Years ago, a psychologist set up a scientific search for the planet's funniest joke.

Over tens of thousands of jokes submitted, with ratings lodged by 350,000 people around the world, he has a clearer understanding than most as to what succeeds and what does not.

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

"But they also need to be bad jokes, jokes that cause us to groan," he continues.

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

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

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

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