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

This one-liner is met by moans that resonate through a storage facility in London.

We're at a joke-testing meeting with a firm that makes supplies for gatherings. Its repertoire features festive crackers.

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

"The success is gauged by the joke by the number of moans and the loudness of the groans around the table," the founder says.

The secret to a great Christmas cracker joke is not the same as a good gag per se. It is all about the context - in this instance, the shared laughter of the holiday dinner table with grandparents, children and possibly friends.

"You want the joke to be something that brings the eight-year-old in harmony with the 80-year-old," she adds.

The Neuroscience Of Shared Laughter

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

"So when you are chuckling with people around the holiday table you are dropping into what's very likely a really primordial mammal social sound," explains a professor.

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

Scientists have discovered that a absence of such social exchanges can significantly damage mental and physical well-being.

"The people you talk to, and laugh with, it results in increased amounts of 'happy chemical' release," she continues.

These natural chemicals are the brain's "feel-good compounds" and are produced both to alleviate stress and pain and in reaction to pleasurable experiences, such as laughing with loved ones over a particularly awful Christmas cracker gag.

"You're not just chuckling at a foolish joke with a Christmas cracker," the expert says. "You are in fact doing a lot of the really vital task of building, preserving the connections you have with those you love."

Which Occurs Inside the Mind?

But what is truly taking place inside the brain when we listen to a joke?

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

Using brain scanning technology, a kind of neural imager which indicates which parts of the brain are working harder, researchers have been able to chart the areas that get more blood flow.

Testing entails imaging the minds of healthy participants and then exposing them to a collection of humorous words, paired with either a neutral sound, or pre-recorded laughter.

"During the study we got a really interesting pattern of activation," notes the neuroscientist.

A joke stimulates not just the parts of the brain responsible for hearing and interpreting speech, but also neural regions associated with both preparation and starting motion and those linked to sight and recall.

Put all of this together, and people hearing a joke have a complex series of neural responses that underpin the laughter we experience.

The Contagious Nature of Chuckles

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

"This was in areas of the brain that you would use to move your expression into a smile or a chuckle," she explains.

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

Amusement, according to the expert, can be infectious.

So what does this mean for the laughter heard at a Christmas table?

"People laugh more when you are familiar with others," she says, "and you laugh more when you like them or love them."

When it comes to Christmas cracker jokes, she says, the feel-good effect is more likely to be triggered not by the gag in itself, but from the response to it.

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

The Search for the Perfect Festive Pun

Is it possible to find the perfect gag?

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

In 2001, a psychologist set up a research project for the planet's funniest joke.

More than 40,000 gags later, with ratings 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 ideal Christmas cracker pun must be brief, he says.

"But they also be poor jokes, jokes that cause us to moan," he adds.

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

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

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

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