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

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

This describes a humor-evaluation session with a firm that makes supplies for social events. Its catalogue features festive crackers.

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

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

The key to a good Christmas cracker pun is not the identical as a stand-up gag in itself. It is all about the setting - in this case, the shared amusement of the holiday meal with elders, children and possibly friends.

"You want the joke to be a thing that unites the eight-year-old in harmony with the 80-year-old," she states.

The Science Of Communal Laughter

Gathering to enjoy communal amusement is not only ancient, scientists say, it is probably to be pre-human.

"So when you are laughing with others at the Christmas dinner you are engaging in what's almost certainly a truly ancient mammal social vocalisation," says a neuroscience expert.

Shared laughter, she explains, helps make and maintain social connections between individuals.

Scientists have discovered that a lack of such interactions can seriously harm both psychological and bodily well-being.

"The people you converse with, and laugh with, it leads to enhanced levels of 'happy chemical' release," the professor continues.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce stress and pain and in reaction to enjoyable experiences, such as chuckling with loved ones over a particularly awful festive cracker joke.

"It's not simply laughing at a silly joke with a Christmas cracker," the expert says. "You are in fact doing a lot of the really vital work of building, preserving the social bonds you have with the people you love."

Which Happens In the Mind?

But what is truly happening inside the mind when we listen to a gag?

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

Employing brain scanning technology, a type of neural imager which indicates which parts of the brain are more active, researchers have been able to chart the regions that receive more blood.

Testing involves scanning the minds of volunteer subjects and then subjecting them to a database of funny phrases, accompanied by either a neutral sound, or recorded laughter.

"In the scanner we got a very interesting activation pattern of activation," says the neuroscientist.

A joke activates not just the areas of the brain responsible for hearing and interpreting speech, but also brain regions associated with both preparation and starting motion and those involved in sight and memory.

Combine all of this together, and people hearing a joke have a complex series of neural reactions that underpin the laughter we hear.

The Contagious Nature of Laughter

Scientists found that when a funny word is paired with chuckles there is a stronger response in the brain than the same phrase when accompanied by a neutral sound.

"This activation occurred in parts of the brain that you would employ to move your expression into a grin or a laugh," the professor says.

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

Laughter, according to the expert, can be contagious.

So what does this imply for the laughter heard around a holiday table?

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

When it comes to festive cracker puns, she explains, the positive effect is more probable to be caused not by the gag itself, but from the reaction to it.

"It's the laughter. The gag is the dreadful Christmas cracker pun, and it's just a reason to laugh as a group."

The Search for the Ideal Festive Pun

Is it possible to discover the ultimate joke?

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

Years ago, a psychologist established a research project for the world's most humorous gag.

Over tens of thousands of gags later, with scores provided by hundreds of thousands of participants globally, he has a clearer understanding than many as to what succeeds and what does not.

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

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

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

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

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

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