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

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

We're at a humor-evaluation meeting with a firm that makes supplies for gatherings. Its repertoire includes Christmas crackers.

The firm's owner grins, almost apologetically at the joke. But the joke has made the cut and will appear in upcoming crackers.

"You measure the joke by the volume of moans and the loudness of the groans around the table," the founder explains.

The secret to a great holiday cracker joke 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 elders, children and potentially friends.

"You want the gag to be a thing that brings the eight-year-old together with the 80-year-old," she states.

The Neuroscience Of Shared Amusement

Coming together to experience communal amusement is not only ancient, scientists say, it is probably to be pre-human.

"Therefore when you are chuckling with others at the Christmas table you are engaging in what's very likely a truly primordial mammal play sound," says a professor.

Shared laughter, she says, helps make and maintain social bonds between people.

Scientists have discovered that a lack of these social exchanges can seriously harm both psychological and bodily health.

"Those you converse with, and laugh with, it results in enhanced amounts of 'happy chemical' release," the professor adds.

Endorphins are the body's "happy chemicals" and are produced both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as laughing with friends over a particularly awful Christmas cracker gag.

"It's not simply laughing at a foolish joke with a Christmas cracker," she says. "You are in fact performing a lot of the truly vital work of making, maintaining the connections you have with the people you love."

Which Occurs Inside the Brain?

But what is truly taking place within the mind when we hear a gag?

An awful lot occurs in response to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which areas of the mind are working harder, scientists have been able to chart the regions that get more blood flow.

Testing involves scanning the brains of healthy participants and then subjecting them to a database of humorous words, accompanied by either a neutral sound, or recorded chuckles.

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

A joke stimulates not just the areas of the mind in charge of auditory processing and interpreting speech, but also brain regions associated with both preparation and starting motion and those linked to sight and recall.

Combine these elements as a whole, and people hearing a pun have a complex set of neural responses that support the amusement we experience.

The Contagious Power of Laughter

Scientists found that when a humorous word is combined with chuckles there is a greater reaction in the brain than the identical phrase when followed by a neutral sound.

"This activation occurred in parts of the mind that you would use to move your face into a smile or a laugh," the professor says.

It means we are not just responding to funny jokes, they are responding to the laughter that follows them.

Amusement, says the professor, can be infectious.

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

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

When it comes to festive cracker puns, she says, the feel-good effect is more likely to be caused not by the gag itself, but from the response to it.

"It's the laughter. The joke is the terrible holiday cracker pun, and it's just a reason to laugh as a group."

The Quest for the Perfect Cracker Joke

Will we ever discover the perfect gag?

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

In 2001, a psychologist set up a research search for the planet's funniest gag.

Over tens of thousands of gags later, with ratings provided by hundreds of thousands of participants around the world, he has a better idea than many as to what works and what fails.

The perfect festive cracker pun must be brief, he explains.

"But they also be bad gags, puns that cause us to groan," he adds.

The increasingly "terrible" the gag, he states the more effective.

"This is because if no-one finds it funny – it's the joke's fault, not yours.

"What's interesting about the Christmas cracker jokes is that not one person find them funny.

"That's a common experience around the table and I believe it's lovely."