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

This joke is greeted with groans that echo through a warehouse in London.

We're at a joke-testing meeting with a firm that produces supplies for gatherings. Its repertoire includes Christmas crackers.

The firm's founder grins, almost apologetically at the gag. But the pun has made the cut and will appear in future crackers.

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

The key to a great Christmas cracker pun is not the same as a good joke in itself. It is all about the setting - in this case, the shared amusement of the Christmas dinner table with grandparents, children and potentially friends.

"You want the gag to be something that brings the child in harmony with the 80-year-old," she states.

The Neuroscience Of Shared Amusement

Coming together to experience shared amusement is not only ancient, scientists argue, it is likely to be older than humanity.

"Therefore when you are laughing with people around the Christmas table you are engaging in what's almost certainly a really ancient mammal play vocalisation," says a professor.

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

Researchers have found that a lack of these social exchanges can significantly harm both psychological and bodily health.

"The people you talk to, and share laughter with, it results in increased amounts of 'happy chemical' uptake," she adds.

Endorphins are the body's "feel-good compounds" and are released both to reduce tension and discomfort and in response to enjoyable experiences, such as laughing with friends over a truly awful festive cracker joke.

"It's not simply chuckling at a silly joke with a holiday cracker," the expert says. "You are actually doing a lot of the really vital task of building, preserving the connections you have with the people you care about."

What Happens In the Mind?

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

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

Employing brain scanning technology, a type of neural imager which shows which parts of the mind are working harder, researchers have been able to map the areas that receive more blood.

Testing entails scanning the brains of healthy participants and then subjecting them to a collection of humorous words, paired with either a neutral sound, or recorded laughter.

"During the study we got a really interesting activation pattern of activation," says the professor.

A gag stimulates not just the parts of the brain in charge of hearing and understanding language, but also neural areas associated with both planning and initiating motion and those involved in sight and memory.

Combine all of this together, and individuals listening to a pun have a sophisticated set of neural responses that underpin the amusement we experience.

The Contagious Nature of Chuckles

Scientists found that when a humorous word is combined with laughter there is a stronger reaction in the brain than the identical word when accompanied by a non-emotional sound.

"This activation occurred in areas of the mind that you would employ to move your expression into a smile or a chuckle," the professor says.

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

Laughter, according to the professor, can be infectious.

So what does this mean for the chuckles found at a Christmas table?

"You laugh harder when you are familiar with others," she says, "and laughter increases further when you are fond of them or love them."

When it comes to Christmas cracker puns, she says, the feel-good factor is more likely to be triggered not by the gag itself, but from the reaction to it.

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

The Search for the Perfect Festive Pun

Is it possible to discover the perfect joke?

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

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

More than 40,000 jokes submitted, with scores provided by hundreds of thousands of people around the world, he has a better idea than many as to what succeeds and what does not.

The ideal festive cracker joke must be short, he explains.

"But they also be poor gags, puns that make us moan," he adds.

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

"The reason is that if nobody laughs – it's the gag's fault, not yours.

"What's interesting about the holiday cracker puns is that none of us find them funny.

"That's a common experience around the gathering and I think it's lovely."