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

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

We're at a joke-testing session with a firm that makes products for social events. Its catalogue includes festive crackers.

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

"You measure the gag by the volume of moans and the intensity of the groans at the table," the founder explains.

The key to a great holiday cracker joke is not the same as a good gag in itself. It is entirely about the context - in this case, the communal amusement of the Christmas dinner table with elders, kids and potentially neighbours.

"You want the gag to be a thing that unites the child together with the 80-year-old," she adds.

The Science Of Shared Laughter

Gathering to enjoy communal amusement is not only nothing new, experts argue, it is probably to be pre-human.

"So when you are laughing with people at the holiday dinner you are engaging in what's very likely a truly ancient mammal social sound," explains a neuroscience expert.

Communal amusement, she says, helps make and maintain social connections between individuals.

Scientists have found that a absence of these social exchanges can significantly damage mental and physical health.

"Those you converse with, and share laughter with, it leads to increased levels of 'happy chemical' release," the professor continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to pleasurable experiences, such as chuckling with friends over a truly awful festive cracker joke.

"You're not just chuckling at a silly pun with a Christmas cracker," she states. "You are in fact doing a lot of the truly important work of building, preserving the connections you have with those you care about."

Which Happens Inside the Mind?

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

An awful lot happens in response to comedy, it turns out.

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

Testing involves scanning the brains of healthy subjects and then exposing them to a collection of funny phrases, accompanied by either a non-emotional sound, or pre-recorded chuckles.

"During the study we observed a very interesting activation pattern of neural activity," notes the professor.

A joke stimulates not just the areas of the brain responsible for auditory processing and interpreting language, but also brain areas associated with both preparation and starting movement and those linked to vision and memory.

Put these elements together, and individuals hearing a joke have a complex set of brain reactions that support the amusement we hear.

The Infectious Nature of Laughter

Scientists discovered that when a funny word is paired with chuckles there is a greater response in the mind than the same phrase when followed by a neutral sound.

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

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

Amusement, according to the expert, can be infectious.

So what does this imply for the chuckles heard around a Christmas table?

"People laugh more when you are familiar with others," she notes, "and laughter increases further when you like them or care for them."

When it comes to festive cracker jokes, she explains, the positive effect is more probable to be caused not by the gag itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Will we ever find the ultimate gag?

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

Years ago, a psychologist set up a scientific search for the world's funniest joke.

More than tens of thousands of jokes submitted, with ratings provided by 350,000 participants globally, he has a clearer understanding than many as to what works and what fails.

The perfect festive cracker joke must be short, he says.

"But they also need to be poor jokes, jokes that cause us to groan," he adds.

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

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

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

"That's a shared moment at the table and I think it's wonderful."