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"What was the price did Father Christmas's sleigh cost? Zero, it was on the house."

This joke is greeted with groans that resonate through a storage facility in the capital.

This describes a joke-testing meeting with a firm that makes supplies for social events. Its catalogue features Christmas crackers.

The firm's owner smiles, almost sheepishly at the gag. But the pun has been selected and will feature in future crackers.

"The success is gauged by the gag by the volume of moans and the intensity of the groans around the table," the founder explains.

The key to a good Christmas cracker pun is not the same as a stand-up gag in itself. It is all about the context - in this case, the communal amusement of the Christmas meal with elders, children and possibly friends.

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

The Neuroscience Behind Shared Laughter

Coming together to enjoy communal amusement is not only nothing new, scientists argue, it is probably to be pre-human.

"Therefore when you are chuckling with people at the holiday table you are dropping into what's almost certainly a truly primordial mammalian play vocalisation," says a neuroscience expert.

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

Scientists have discovered that a lack of such interactions can seriously damage mental and physical health.

"The people you talk to, and share laughter with, it leads to enhanced amounts of 'happy chemical' release," she continues.

Endorphins are the body's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to pleasurable experiences, such as chuckling with loved ones over a particularly awful festive cracker joke.

"It's not simply laughing at a silly pun with a Christmas cracker," the expert states. "You are in fact performing a lot of the truly vital task of making, maintaining the connections you have with those you care about."

Which Happens In the Mind?

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

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

Employing brain scanning technology, a kind of brain scanner which indicates which parts of the mind are more active, researchers have been able to map the areas that receive more blood flow.

Testing entails imaging the brains of volunteer participants and then exposing them to a database of funny words, accompanied by either a neutral sound, or recorded laughter.

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

A gag activates not just the parts of the brain in charge of hearing and interpreting speech, but also neural regions involved in both planning and initiating movement and those involved in vision and memory.

Put all of this together, and people hearing a joke have a sophisticated series of neural responses that support the amusement we hear.

The Infectious Power of Chuckles

Researchers discovered that when a humorous phrase is paired with chuckles there is a stronger reaction in the mind than the identical word when accompanied by a non-emotional sound.

"This activation occurred in areas of the brain that you would use to move your expression into a smile or a chuckle," the professor says.

It means people are not just reacting to funny jokes, they are reacting to the laughter that accompanies them.

Laughter, according to the expert, can be infectious.

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

"You laugh more when you know others," she notes, "and laughter increases further when you are fond of them or love them."

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

"It's the laughter. The gag is the dreadful Christmas cracker pun, and it's just a pretext to laugh as a group."

The Quest for the Perfect Festive Pun

Will we ever discover the perfect gag?

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

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

More than tens of thousands of jokes submitted, with ratings lodged by 350,000 people globally, he has a better idea than most as to what works and what does not.

The ideal Christmas cracker joke must be brief, he explains.

"They must also need to be bad gags, jokes that cause us to moan," he adds.

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

"The reason is that if nobody laughs – it's the gag's shortcoming, not your own.

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

"It creates a common moment around the gathering and I believe it's wonderful."