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

This quip is met by groans that echo through a warehouse in the capital.

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

The company's owner smiles, nearly sheepishly at the gag. But the joke has been selected and will feature in upcoming crackers.

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

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

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

The Neuroscience Of Shared Amusement

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

"Therefore when you are laughing with others at the Christmas table you are dropping into what's almost certainly a really ancient mammal play vocalisation," says a professor.

Communal amusement, she explains, aids in make and maintain social connections between individuals.

Researchers have found that a absence of such interactions can seriously damage both psychological and bodily well-being.

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

Endorphins are the brain's "feel-good compounds" and are produced both to alleviate stress and pain and in response to enjoyable experiences, such as chuckling with loved ones over a truly terrible Christmas cracker gag.

"You're not just laughing at a foolish joke with a Christmas cracker," she states. "You are actually doing a lot of the really vital task of making, maintaining the social bonds you have with the people you care about."

Which Occurs Inside the Brain?

But what is actually taking place inside the mind when we listen to a joke?

An awful lot happens in reaction to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which areas of the mind are working harder, scientists have been able to chart the regions that receive more blood flow.

The research entails imaging the brains of healthy participants and then exposing them to a collection of funny words, paired with either a non-emotional sound, or recorded laughter.

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

A joke stimulates not just the parts of the mind in charge of auditory processing and interpreting speech, but also neural areas involved in both preparation and initiating movement and those involved in sight and recall.

Put these elements together, and people hearing a joke have a complex series of neural reactions that support the amusement we hear.

The Contagious Nature of Laughter

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

"This activation occurred in parts of the mind that you would employ to move your face into a smile or a laugh," the professor explains.

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

Amusement, according to the professor, can be infectious.

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

"People laugh more when you are familiar with others," she notes, "and you laugh further 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 caused not by the gag itself, but from the reaction to it.

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

The Quest for the Ideal Festive Pun

Will we ever discover the perfect joke?

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

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

More than tens of thousands of gags later, with scores provided by hundreds of thousands of people globally, he has a clearer understanding than many as to what succeeds and what fails.

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

"They must also need to be poor gags, puns that cause us to groan," he continues.

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

"This is because if nobody finds it funny – it's the gag's shortcoming, not yours.

"The fascinating part about the holiday cracker jokes is that not one person find them funny.

"That's a shared moment at the table and I believe it's lovely."