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

This quip is greeted with groans that resonate through a warehouse in London.

We're at a joke-testing session with a firm that makes products for social events. Its repertoire features Christmas crackers.

The company's owner smiles, almost apologetically at the gag. But the pun has been selected and will appear in future crackers.

"You measure the joke by the number of groans and the loudness of the groans around the table," she says.

The secret to a great Christmas cracker joke is not the identical as a good gag in itself. It is entirely about the setting - in this case, the shared laughter of the holiday dinner table with grandparents, kids and potentially friends.

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

The Neuroscience Of Communal Amusement

Gathering to experience communal amusement is not only ancient, scientists say, it is likely to be pre-human.

"So when you are laughing with people around the holiday table you are engaging in what's almost certainly a really ancient mammal play vocalisation," says a professor.

Shared amusement, she says, aids in make and maintain social bonds between individuals.

Researchers have found that a absence of these social exchanges can significantly damage both psychological and bodily well-being.

"The people you talk to, and laugh with, it results in enhanced levels of endorphin uptake," the professor adds.

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

"It's not simply chuckling at a foolish pun with a Christmas cracker," the expert says. "You are actually performing a lot of the truly 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 inside the brain when we hear a joke?

A tremendous amount happens in response to humour, it transpires.

Using brain scanning technology, a type of brain scanner which shows which areas of the mind are more active, scientists have been able to map the regions that get more blood flow.

Testing entails imaging the minds of healthy participants and then subjecting them to a collection of humorous words, accompanied by either a non-emotional sound, or pre-recorded chuckles.

"In the scanner we observed a really interesting activation pattern of neural activity," says the neuroscientist.

A joke stimulates not just the parts of the brain in charge of auditory processing and understanding language, but also neural regions involved in both preparation and starting movement and those involved in vision and memory.

Combine all of this together, and individuals hearing a pun have a sophisticated set of brain responses that underpin the laughter we hear.

The Contagious Nature of Laughter

Scientists discovered that when a humorous word is paired with laughter there is a greater response in the brain than the same phrase when accompanied by a neutral sound.

"This was in areas of the mind that you would employ to contort your face into a grin or a laugh," the professor explains.

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

Amusement, says the professor, can be infectious.

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

"People laugh more when you know people," she notes, "and you laugh further when you are fond of them or love them."

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

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

The Search for the Perfect Cracker Joke

Will we ever find the perfect gag?

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

In 2001, a psychologist established a scientific project for the world's funniest gag.

More than tens of thousands of jokes submitted, with ratings lodged by hundreds of thousands of participants globally, he has a clearer idea than most as to what works and what fails.

The perfect festive cracker joke must be brief, he explains.

"They must also be bad gags, puns that cause us to moan," he continues.

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

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

"What's interesting about the holiday cracker puns is that not one person considers them humorous.

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