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

This one-liner is met by moans that echo through a warehouse in London.

We're at a joke-testing session with a firm that produces products for gatherings. Its catalogue includes Christmas crackers.

The firm's founder smiles, almost sheepishly at the joke. But the joke has been selected and will appear in future crackers.

"The success is gauged by the gag by the volume of moans and the intensity of the groans at the table," she says.

The key to a good Christmas cracker joke is not the same as a stand-up joke per se. It is all about the context - in this instance, the shared amusement of the Christmas meal with grandparents, children and potentially friends.

"The goal is for the joke to be a thing that unites the child in harmony with the 80-year-old," she states.

The Neuroscience Behind Communal Amusement

Gathering to enjoy shared laughter is not only nothing new, scientists say, it is probably to be pre-human.

"So when you are laughing with others at the Christmas table you are engaging in what's very likely a really primordial mammalian play vocalisation," says a neuroscience expert.

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

Scientists have found that a absence of these interactions can seriously harm both psychological and bodily health.

"The people you talk to, and laugh with, it leads to increased levels of endorphin uptake," the professor continues.

Endorphins are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in response to pleasurable experiences, such as chuckling with loved ones over a truly awful Christmas cracker joke.

"It's not simply chuckling at a foolish joke with a Christmas cracker," the expert states. "You are actually doing a lot of the truly vital task of building, preserving the social bonds you have with those you care about."

Which Happens Inside the Mind?

But what is truly taking place within the brain when we hear a gag?

A tremendous amount happens in reaction to comedy, it turns out.

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which parts of the mind are more active, scientists have been able to chart the areas that get more blood flow.

The research involves scanning the minds of volunteer subjects and then exposing them to a database of funny words, paired with either a non-emotional sound, or recorded chuckles.

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

A gag activates not just the parts of the mind in charge of auditory processing and understanding language, but also neural regions associated with both preparation and starting movement and those involved in sight and recall.

Combine these elements as a whole, and individuals listening to a pun have a complex set of brain responses that underpin the laughter we hear.

The Infectious Nature of Chuckles

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

"This was in parts of the mind that you would employ to contort your face into a grin or a laugh," she says.

It means people are not just reacting to humorous words, they are reacting to the laughter that follows them.

Laughter, says the professor, can be contagious.

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

"You 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 jokes, she explains, the positive factor is more probable to be caused not by the gag in itself, but from the reaction to it.

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

The Quest for the Ideal Festive Pun

Will we ever discover the perfect joke?

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

In 2001, a professor set up a scientific search for the world's funniest gag.

Over tens of thousands of gags later, with scores provided by hundreds of thousands of people globally, he has a better understanding than most as to what works and what does not.

The ideal festive cracker pun must be short, he says.

"They must also need to be poor jokes, jokes that cause us to groan," he adds.

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

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

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

"That's a common experience at the gathering and I believe it's wonderful."