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

This quip is greeted with groans that echo through a storage facility in London.

We're at a humor-evaluation meeting with a company that makes products for gatherings. Its catalogue features Christmas crackers.

The company's founder grins, almost sheepishly at the gag. But the joke has made the cut and will appear in upcoming crackers.

"The success is gauged by the joke by the number of groans and the loudness of the groans at the table," the founder explains.

The secret to a good Christmas cracker joke is not the same as a good joke in itself. It is all about the context - in this instance, the communal laughter of the Christmas meal with grandparents, kids and possibly neighbours.

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

The Science Behind Communal Amusement

Gathering to enjoy communal amusement is not only ancient, 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 truly primordial mammal social vocalisation," says a neuroscience expert.

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

Researchers have found that a absence of such social exchanges can seriously harm mental and physical well-being.

"Those you talk to, and share laughter with, it leads to enhanced levels of endorphin release," she continues.

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

"You're not just chuckling at a silly joke with a holiday cracker," she states. "You are actually doing a lot of the really vital work of making, maintaining the connections you have with the people you love."

Which Happens In the Brain?

But what is actually happening within the mind when we listen to a joke?

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

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which parts of the mind are working harder, scientists have been able to chart the regions that receive more blood flow.

Testing involves imaging the brains of volunteer subjects and then exposing them to a database of funny words, accompanied by either a non-emotional sound, or recorded chuckles.

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

A gag stimulates not just the areas of the brain responsible for auditory processing and understanding speech, but also brain areas associated with both planning and starting movement and those linked to sight and memory.

Put these elements together, and individuals listening to a joke have a complex set of neural responses that underpin the amusement we experience.

The Contagious Nature of Chuckles

Scientists found that when a funny word is paired with laughter there is a stronger response in the brain than the identical phrase when followed by a neutral sound.

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

It means we are not just responding to humorous words, they are responding to the amusement that accompanies them.

Amusement, according to the professor, can be contagious.

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

"People laugh harder when you know people," she notes, "and laughter increases more when you like them or love them."

When it comes to festive cracker jokes, she explains, the positive factor is more probable to be caused not by the joke in itself, but from the reaction to it.

"It's the laughter. The gag is the terrible holiday cracker joke, and it's just a pretext to chuckle together."

The Search for the Perfect Cracker Joke

Will we ever find the ultimate gag?

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

In 2001, a psychologist established a scientific search for the planet's most humorous joke.

Over tens of thousands of jokes later, with ratings lodged by hundreds of thousands of participants globally, he has a clearer idea than most as to what works and what does not.

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

"But they also be poor gags, puns that make us groan," he continues.

The more "terrible" the joke, he says the better.

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

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

"It creates a shared moment around the table and I believe it's wonderful."