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"How much did Santa's sleigh cost? Nothing, it was on the house."

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

This describes a joke-testing meeting with a company that makes products for social events. Its catalogue includes festive crackers.

The company's owner smiles, almost apologetically at the joke. But the joke has been selected and will feature in upcoming crackers.

"The success is gauged by the joke by the number of moans and the loudness of the groans at the table," she explains.

The key to a great holiday cracker pun is not the same as a good joke per se. It is entirely about the context - in this instance, the communal amusement of the Christmas dinner table with grandparents, children and possibly neighbours.

"You want the gag to be something that unites the eight-year-old in harmony with the grandparent," she adds.

The Neuroscience Behind Shared Laughter

Coming together to experience shared amusement is not only ancient, experts argue, it is likely to be pre-human.

"Therefore when you are chuckling with people at the Christmas dinner you are engaging in what's almost certainly a truly primordial mammalian play sound," explains a professor.

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

Scientists have found that a absence of these social exchanges can significantly damage both psychological and bodily well-being.

"The people you converse with, and share laughter with, it leads to enhanced amounts of endorphin uptake," she continues.

Endorphins are the body's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to enjoyable experiences, such as laughing with friends over a particularly terrible Christmas cracker gag.

"It's not simply laughing at a foolish joke with a Christmas cracker," the expert says. "You are in fact performing a lot of the truly important task of building, preserving the social bonds you have with the people you love."

What Happens In the Brain?

But what is actually taking place within the mind when we hear a joke?

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

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which shows which areas of the mind are more active, researchers have been able to map the regions that receive more blood.

Testing entails scanning the minds of volunteer subjects and then subjecting them to a collection of humorous phrases, accompanied by either a non-emotional sound, or pre-recorded chuckles.

"In the scanner we got a really fascinating activation pattern of neural activity," notes the neuroscientist.

A joke stimulates not just the areas of the mind responsible for hearing and interpreting speech, but also brain regions associated with both planning and initiating motion and those involved in vision and recall.

Combine these elements as a whole, and people hearing a joke have a complex series of neural responses that underpin the laughter we hear.

The Infectious Nature of Chuckles

Researchers found that when a funny word is paired with laughter there is a greater reaction in the mind than the same phrase when accompanied by a neutral sound.

"This activation occurred in areas of the mind that you would use to contort your face into a smile or a laugh," she says.

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

Amusement, according to the expert, can be contagious.

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

"You laugh more when you are familiar with others," she says, "and laughter increases more when you are fond of them or care for them."

When it comes to festive cracker puns, she says, the positive effect is more probable to be caused not by the joke itself, but from the response to it.

"The laughter is key. The gag is the dreadful holiday cracker pun, and it's just a pretext to chuckle as a group."

The Search for the Perfect Festive Pun

Will we ever discover the perfect gag?

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

In 2001, a professor set up a scientific search for the world's most humorous gag.

More than 40,000 jokes submitted, with scores provided by hundreds of thousands of participants around the world, he has a better 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 gags, puns that make us moan," he continues.

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

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

"The fascinating part about the Christmas cracker puns is that none of us considers them humorous.

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