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

This quip is met by groans that resonate through a storage facility in the capital.

This describes a joke-testing meeting with a company that makes products for gatherings. Its repertoire includes festive crackers.

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

"The success is gauged by the gag by the volume of groans and the loudness of the groans at the table," she says.

The key to a good Christmas cracker pun is not the identical as a good gag per se. It is all about the setting - in this instance, the shared laughter of the holiday dinner table with elders, kids and possibly neighbours.

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

The Science Of Communal Laughter

Coming together to experience communal laughter is not only ancient, experts argue, it is probably to be pre-human.

"So when you are laughing with others at the Christmas dinner you are engaging in what's very likely a really ancient mammal social vocalisation," explains a professor.

Communal amusement, she explains, helps forge and strengthen social connections between people.

Researchers have discovered that a lack of such interactions can seriously damage mental and physical health.

"The people you talk to, and share laughter with, it results in enhanced amounts of endorphin release," she adds.

These natural chemicals are the body's "happy chemicals" and are released both to alleviate stress and pain and in response to pleasurable experiences, such as chuckling with friends over a particularly terrible Christmas cracker gag.

"It's not simply laughing at a foolish joke with a holiday cracker," the expert says. "You are actually doing a lot of the truly important work of building, preserving the connections you have with those you love."

Which Happens In the Brain?

But what is actually taking place inside the brain when we hear a gag?

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the mind are more active, scientists have been able to chart the regions that receive more blood flow.

The research involves imaging the brains of volunteer participants and then subjecting them to a collection of humorous words, accompanied by either a non-emotional sound, or recorded chuckles.

"During the study we observed a very interesting activation pattern of activation," notes the professor.

A gag stimulates not just the areas of the brain responsible for hearing and interpreting speech, but also neural regions associated with both planning and initiating movement and those involved in sight and recall.

Put all of this as a whole, and people hearing a joke have a sophisticated set of neural responses that support the laughter we hear.

The Infectious Power of Laughter

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

"This was in areas of the mind that you would use to contort your expression into a smile or a laugh," the professor explains.

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

Amusement, says the expert, can be contagious.

So what does this imply for the chuckles found around a Christmas table?

"You laugh more when you know others," she says, "and you laugh more when you are fond of them or love them."

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

"It's the laughter. The joke is the terrible Christmas cracker pun, and it's just a pretext to laugh together."

The Quest for the Ideal Cracker Joke

Is it possible to discover the ultimate gag?

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

Years ago, a psychologist set up a scientific project for the world's funniest joke.

More than tens of thousands of gags later, with ratings provided by 350,000 people around the world, he has a clearer idea than many as to what works and what does not.

The perfect Christmas cracker joke needs to be brief, he says.

"But they also need to be bad gags, jokes that make us groan," he adds.

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

"The reason is that if no-one finds it funny – it's the gag's shortcoming, not your own.

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

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