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"How much did Santa's sleigh cost? Zero, it was on the house."

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

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

The firm's founder smiles, almost sheepishly at the joke. But the joke has made the cut and will appear in future crackers.

"You measure the gag by the volume of groans and the intensity of the groans around the table," she explains.

The key to a good Christmas cracker joke is not the identical as a good joke per se. It is entirely about the context - in this instance, the communal laughter of the Christmas dinner table with elders, kids and potentially friends.

"The goal is for the joke to be something that brings the child together with the grandparent," she adds.

The Neuroscience Of Shared Amusement

Coming together to experience communal laughter is not only nothing new, experts say, it is probably to be older than humanity.

"Therefore when you are laughing with others at the holiday dinner you are engaging in what's very likely a truly ancient mammalian play sound," explains a neuroscience expert.

Communal laughter, she explains, helps make and maintain social bonds between individuals.

Scientists have discovered that a lack of such social exchanges can seriously damage mental and physical health.

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

These natural chemicals are the body's "happy chemicals" and are produced both to alleviate tension and discomfort and in response to pleasurable activities, such as laughing with loved ones over a particularly terrible festive cracker gag.

"It's not simply laughing at a foolish joke with a holiday cracker," she says. "You are in fact performing a lot of the really important task of building, preserving the social bonds you have with those you care about."

What Happens Inside the Brain?

But what is actually taking place within the mind when we listen to a gag?

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which areas of the brain are more active, scientists have been able to chart the areas that get more blood.

Testing involves imaging the brains of healthy subjects and then exposing them to a database of funny words, accompanied by either a non-emotional sound, or recorded laughter.

"In the scanner we observed a very interesting activation pattern of neural activity," says the neuroscientist.

A gag stimulates not just the areas of the mind responsible for hearing and interpreting speech, but also brain areas associated with both planning and initiating movement and those linked to vision and memory.

Combine these elements together, and people listening to a joke have a complex series of brain responses that underpin the laughter we hear.

The Contagious Nature of Laughter

Researchers found that when a humorous word is combined with chuckles there is a greater response in the brain than the same word when followed by a non-emotional sound.

"This activation occurred in parts of the brain that you would employ to move your face into a smile or a laugh," she says.

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

Amusement, according to the expert, can be infectious.

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

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

When it comes to festive cracker puns, she says, the positive factor is more probable to be triggered not by the gag in itself, but from the reaction to it.

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

The Search for the Perfect Cracker Joke

Will we ever find the ultimate joke?

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

Years ago, a professor set up a scientific search for the world's funniest gag.

More than 40,000 gags later, with ratings lodged by 350,000 people around the world, he has a clearer idea than many as to what works and what fails.

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

"But they also be bad gags, jokes that cause us to groan," he continues.

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

"This is because if no-one laughs – it's the gag's fault, not yours.

"The fascinating part about the holiday cracker jokes is that none of us find them humorous.

"It creates a shared experience at the gathering and I think it's wonderful."