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"What was the price did Father Christmas's sled cost? Nothing, it was on the house."

This one-liner is greeted with groans that echo through a storage facility in the capital.

We're at a humor-evaluation session with a company that makes products for gatherings. Its repertoire includes Christmas crackers.

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

"You measure the gag by the number of groans and the loudness of the groans around the table," the founder explains.

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

"You want the gag to be something that brings the eight-year-old together with the grandparent," she states.

The Neuroscience Of Shared Amusement

Coming together to enjoy shared laughter is not only ancient, experts say, it is likely to be pre-human.

"Therefore when you are laughing with people around the Christmas table you are dropping into what's almost certainly a really ancient mammal social vocalisation," explains a professor.

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

Scientists have discovered that a absence of such interactions can significantly harm both psychological and bodily health.

"The people you converse with, and laugh with, it leads to enhanced amounts of endorphin release," she continues.

Endorphins are the brain's "happy chemicals" and are produced both to reduce stress and pain and in reaction to enjoyable experiences, such as chuckling with friends over a particularly awful festive cracker gag.

"It's not simply laughing at a foolish pun with a holiday cracker," she states. "You are in fact doing a lot of the truly important work of building, preserving the social bonds you have with the people you love."

What Occurs In the Mind?

But what is actually happening inside the mind when we listen to a gag?

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

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which parts of the brain are more active, researchers have been able to chart the areas that receive more blood.

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

"In the scanner we got a really interesting pattern of neural activity," says the professor.

A joke activates not just the areas of the mind responsible for auditory processing and interpreting speech, but also neural areas involved in both planning and starting movement and those involved in vision and recall.

Put all of this together, and people listening to a pun have a sophisticated set of brain reactions that underpin the laughter we experience.

The Infectious Power of Chuckles

Scientists found that when a funny word is paired with chuckles there is a stronger reaction 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 expression into a grin or a chuckle," the professor explains.

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

Amusement, according to the professor, can be infectious.

So what does this mean for the laughter found at a holiday table?

"People laugh more when you are familiar with others," she notes, "and you laugh more when you are fond of them or care for them."

When it comes to Christmas cracker puns, she explains, the feel-good effect is more probable to be caused not by the gag itself, but from the reaction to it.

"The laughter is key. The joke is the terrible holiday cracker joke, and it's just a reason to laugh together."

The Quest for the Ideal Festive Pun

Is it possible to discover the ultimate joke?

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

In 2001, a professor established a scientific project for the planet's most humorous gag.

Over 40,000 jokes submitted, with scores provided by 350,000 people around the world, he has a better idea than many as to what succeeds and what fails.

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

"But they also need to be poor gags, puns that cause us to moan," he adds.

The increasingly "terrible" the joke, he says the more effective.

"The reason is that if nobody finds it funny – it's the gag's shortcoming, not yours.

"The fascinating part about the Christmas cracker puns is that not one person considers them humorous.

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