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

This quip is met by moans that resonate through a warehouse in London.

We're at a humor-evaluation session with a firm that produces products for gatherings. Its catalogue features Christmas crackers.

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

"You measure the gag by the number of groans and the intensity of the groans at the table," the founder says.

The secret to a good Christmas cracker joke is not the identical as a good gag in itself. It is entirely about the setting - in this case, the communal amusement of the holiday meal with elders, kids and possibly friends.

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

The Neuroscience Of Shared Laughter

Gathering to enjoy shared laughter is not only nothing new, experts say, it is probably to be older than humanity.

"Therefore when you are chuckling with others around the Christmas dinner you are engaging in what's almost certainly a truly primordial mammal social vocalisation," explains a neuroscience expert.

Communal amusement, she says, aids in make and maintain social bonds between people.

Scientists have discovered that a lack of these interactions can significantly damage mental and physical well-being.

"Those you talk to, and laugh with, it results in increased levels of 'happy chemical' release," the professor continues.

These natural chemicals are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as chuckling with friends over a particularly awful Christmas cracker joke.

"It's not simply chuckling at a silly joke with a Christmas cracker," the expert says. "You are in fact performing a lot of the really important task of making, maintaining the social bonds you have with the people you care about."

Which Happens In the Mind?

But what is truly taking place within the mind when we listen to a gag?

A tremendous amount happens in response to humour, it turns out.

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

Testing involves scanning the minds of healthy participants and then subjecting them to a collection of funny words, accompanied by either a neutral sound, or recorded chuckles.

"In the scanner we observed a really fascinating activation pattern of activation," notes the professor.

A gag activates not just the areas of the brain responsible for auditory processing and understanding speech, but also brain areas associated with both preparation and starting motion and those linked to vision and memory.

Combine these elements as a whole, and individuals listening to a pun have a complex series of brain reactions that support the laughter we experience.

The Contagious Power of Chuckles

Scientists found that when a funny phrase is combined with chuckles there is a stronger response in the mind than the identical phrase when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would employ to contort your expression into a smile or a laugh," the professor says.

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

Amusement, according to the expert, can be contagious.

So what does this imply for the chuckles heard around a holiday gathering?

"People laugh harder when you know others," she says, "and laughter increases further when you are fond of them or care for them."

When it comes to Christmas cracker jokes, she explains, the positive factor is more likely to be caused not by the joke itself, but from the reaction to it.

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the perfect gag?

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

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

More than 40,000 gags submitted, with ratings provided by 350,000 people around the world, he has a clearer understanding than most as to what works and what does not.

The ideal Christmas cracker joke needs to be short, he says.

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

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

"The reason is that if nobody finds it funny – it's the gag's fault, not your own.

"What's interesting about the Christmas cracker puns is that none of us find them funny.

"That's a common moment at the gathering and I think it's lovely."