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

This joke is greeted with moans that echo through a warehouse in London.

This describes a joke-testing session with a company that produces products for social events. Its catalogue features Christmas crackers.

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

"The success is gauged by the gag by the volume of groans and the loudness of the groans at the table," she says.

The secret to a good Christmas cracker pun is not the identical as a stand-up gag per se. It is entirely about the context - in this case, the communal laughter of the Christmas meal with grandparents, children and potentially friends.

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

The Neuroscience Of Communal Amusement

Gathering to enjoy shared amusement is not only nothing new, scientists say, it is likely to be older than humanity.

"So when you are laughing with people around the holiday dinner you are dropping into what's almost certainly a really ancient mammalian social sound," explains a neuroscience expert.

Communal laughter, she says, aids in forge and strengthen social bonds between individuals.

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

"Those you converse with, and share laughter with, it leads to increased amounts of endorphin release," the professor continues.

Endorphins are the body's "happy chemicals" and are released both to alleviate tension and discomfort and in reaction to enjoyable activities, such as laughing with friends over a truly awful festive cracker joke.

"You're not just laughing at a silly joke with a Christmas cracker," she says. "You are in fact performing a lot of the truly important task of making, maintaining the social bonds you have with the people you care about."

What Occurs In the Brain?

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

A tremendous amount happens in response to comedy, it transpires.

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

Testing entails scanning the brains of volunteer subjects and then subjecting them to a collection of funny words, paired with either a neutral sound, or recorded laughter.

"During the study we observed a very fascinating activation pattern of neural activity," notes the professor.

A joke stimulates not just the areas of the mind responsible for auditory processing and understanding speech, but also brain regions involved in both planning and starting motion and those linked to vision and recall.

Put all of this as a whole, and people listening to a joke have a complex set of brain reactions that underpin the laughter we experience.

The Infectious Nature of Chuckles

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

"This was in parts of the mind that you would use to contort your expression into a smile or a laugh," the professor says.

It means we are not just responding to humorous words, they are responding to the amusement that accompanies them.

Laughter, says the professor, can be contagious.

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

"People laugh more when you know people," she says, "and you laugh more when you are fond of them or care for them."

When it comes to Christmas cracker jokes, she explains, the positive effect is more probable to be caused not by the joke in 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 as a group."

The Quest for the Perfect Cracker Joke

Is it possible to discover the ultimate gag?

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

In 2001, a psychologist established a research project for the world's most humorous gag.

More than tens of thousands of gags submitted, with scores lodged 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 joke must be short, he explains.

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

The more "terrible" the gag, he says the better.

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

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

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