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

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

This describes a joke-testing meeting with a firm that makes supplies for gatherings. Its catalogue features festive crackers.

The firm's owner grins, nearly sheepishly at the gag. But the joke has been selected and will feature in upcoming crackers.

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

The secret to a great holiday cracker joke is not the same as a stand-up joke in itself. It is entirely about the setting - in this case, the shared laughter of the Christmas dinner table with elders, children and possibly neighbours.

"You want the joke to be a thing that brings the eight-year-old in harmony with the grandparent," she states.

The Science Behind Communal Laughter

Gathering to experience shared laughter is not only ancient, experts say, it is probably to be older than humanity.

"Therefore when you are chuckling with others at the Christmas table you are engaging in what's very likely a truly ancient mammal social sound," explains a neuroscience expert.

Shared amusement, she says, helps make and maintain social bonds between individuals.

Scientists have discovered that a lack of these interactions can significantly damage both psychological and bodily well-being.

"Those you converse with, and laugh with, it results in increased amounts of endorphin uptake," the professor adds.

Endorphins are the body's "happy chemicals" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as chuckling with loved ones over a particularly awful festive cracker joke.

"It's not simply laughing at a silly pun with a Christmas cracker," the expert states. "You are actually performing a lot of the really vital task of building, preserving the connections you have with the people you care about."

What Happens Inside the Brain?

But what is truly happening inside the mind when we hear a joke?

An awful lot occurs in response to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which parts of the brain are working harder, scientists have been able to chart the regions that receive more blood flow.

Testing involves scanning the minds of healthy participants and then subjecting them to a database of humorous phrases, accompanied by either a non-emotional sound, or pre-recorded laughter.

"During the study we got a very interesting pattern of neural activity," says the neuroscientist.

A joke stimulates not just the parts of the brain responsible for auditory processing and interpreting speech, but also brain regions involved in both preparation and starting motion and those linked to vision and memory.

Combine all of this together, and people listening to a pun have a sophisticated set of brain reactions that support the laughter we hear.

The Contagious Power of Chuckles

Scientists discovered that when a funny phrase is paired with chuckles there is a stronger reaction in the mind than the same word when followed by a non-emotional sound.

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

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

Amusement, says the professor, can be contagious.

So what does this mean for the laughter heard around a Christmas table?

"You laugh more when you know others," she notes, "and you laugh more when you like them or love them."

When it comes to festive cracker puns, she explains, the positive 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 Christmas cracker joke, and it's just a reason to chuckle as a group."

The Search for the Ideal Festive Pun

Will we ever discover the ultimate gag?

Probably not, but that has not stopped experts from trying to.

In 2001, a professor set up a research project for the world's funniest joke.

Over 40,000 jokes later, with ratings provided by hundreds of thousands of people around the world, he has a clearer understanding than many as to what succeeds and what does not.

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

"But they also need to be poor jokes, puns that cause us to moan," he continues.

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

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

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

"It creates a common experience at the table and I believe it's lovely."