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

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

This describes a humor-evaluation meeting with a firm that produces products for social events. Its repertoire includes Christmas crackers.

The company's founder smiles, nearly apologetically at the joke. But the joke has been selected and will appear in upcoming crackers.

"You measure the joke by the volume of moans and the intensity of the groans at the table," she says.

The key to a great Christmas cracker joke is not the same as a stand-up joke per se. It is all about the setting - in this case, the shared amusement of the holiday dinner table with elders, kids and possibly friends.

"You want the gag to be a thing that brings the child together with the grandparent," she states.

The Neuroscience Of Communal Amusement

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

"Therefore when you are laughing with people around the Christmas table you are engaging in what's almost certainly a truly primordial mammal social sound," explains a professor.

Communal amusement, she says, aids in forge and strengthen social connections between people.

Researchers have found that a lack of these interactions can seriously harm mental and physical health.

"Those you converse with, and laugh with, it leads to increased amounts of endorphin release," she adds.

Endorphins are the body's "happy chemicals" and are released both to reduce stress and pain and in response to pleasurable activities, such as chuckling with friends over a truly awful Christmas cracker joke.

"It's not simply laughing at a silly joke with a Christmas cracker," the expert states. "You are actually doing a lot of the really important work of making, maintaining the social bonds you have with the people you love."

Which Happens In the Brain?

But what is actually happening within the brain when we listen to a joke?

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which parts of the mind are more active, researchers have been able to chart the regions that get more blood flow.

Testing entails imaging the minds of volunteer participants and then subjecting them to a collection of funny words, accompanied by either a neutral sound, or recorded laughter.

"In the scanner we observed a really interesting activation pattern of activation," says the neuroscientist.

A joke activates not just the parts of the brain in charge of auditory processing and understanding speech, but also neural areas associated with both preparation and starting movement and those linked to sight and memory.

Combine these elements as a whole, and individuals hearing a joke have a complex set of neural reactions that underpin the laughter we hear.

The Infectious Power of Laughter

Researchers found that when a funny word is combined with laughter there is a stronger reaction in the mind than the same phrase when accompanied by a non-emotional sound.

"This was in areas of the brain that you would employ to move your expression into a grin or a laugh," she explains.

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

Laughter, says the expert, can be contagious.

So what does this mean for the laughter found at a Christmas gathering?

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

When it comes to Christmas cracker jokes, she says, the positive factor is more likely to be caused not by the gag itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Will we ever discover the perfect gag?

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

Years ago, a professor set up a research project for the world's most humorous gag.

More than tens of thousands of jokes later, with scores provided by hundreds of thousands of people around the world, he has a clearer understanding than most as to what succeeds and what fails.

The ideal festive cracker joke must be brief, he says.

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

The increasingly "awful" the joke, he says the better.

"This is because if nobody laughs – it's the joke's fault, not yours.

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

"It creates a common experience around the table and I think it's lovely."