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

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

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

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

"You measure the joke by the number of groans and the intensity of the groans at the table," the founder explains.

The secret to a great holiday cracker joke is not the same as a stand-up gag in itself. It is all about the setting - in this instance, the shared amusement of the Christmas meal with elders, kids and possibly friends.

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

The Neuroscience Behind Shared Amusement

Gathering to experience communal laughter is not only ancient, experts argue, it is likely to be pre-human.

"Therefore when you are chuckling with people at the holiday dinner you are dropping into what's very likely a truly ancient mammalian social vocalisation," explains a neuroscience expert.

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

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

"The people you converse with, and laugh with, it leads to enhanced levels of 'happy chemical' release," she continues.

These natural chemicals are the brain's "feel-good compounds" and are released both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as laughing with loved ones over a truly terrible festive cracker joke.

"You're not just laughing at a silly pun with a holiday cracker," she says. "You are actually doing a lot of the truly important work of building, preserving the social bonds you have with the people you care about."

Which Occurs Inside the Brain?

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

A tremendous amount occurs in reaction to comedy, it transpires.

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which areas of the mind are working harder, scientists have been able to chart the regions that get more blood.

Testing involves scanning the brains of volunteer participants and then subjecting them to a collection of humorous words, paired with either a neutral sound, or pre-recorded laughter.

"During the study we observed a very fascinating pattern of activation," says the professor.

A joke activates not just the parts of the brain responsible for hearing and interpreting language, but also neural regions associated with both planning and initiating motion and those involved in sight and memory.

Put all of this as a whole, and people hearing a pun have a complex series of neural responses that support the laughter we hear.

The Contagious Nature of Laughter

Scientists found that when a funny word is combined with chuckles there is a stronger response in the brain than the identical phrase when followed by a non-emotional sound.

"This activation occurred in areas of the mind that you would employ to move your face into a smile or a chuckle," the professor says.

It indicates people are not just reacting to humorous jokes, they are responding to the laughter that follows them.

Amusement, says the expert, can be contagious.

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

"People laugh more when you are familiar with others," she says, "and you laugh further when you are fond of them or love them."

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

"It's the laughter. The gag is the terrible holiday cracker joke, and it's just a reason to laugh as a group."

The Search for the Ideal Cracker Joke

Will we ever discover the perfect joke?

Probably not, but that has not prevented experts from trying to.

Years ago, a psychologist established a research project for the planet's most humorous joke.

More than 40,000 gags later, with scores lodged by hundreds of thousands of people around the world, he has a clearer understanding than most as to what succeeds and what does not.

The perfect Christmas cracker joke needs to be short, he explains.

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

The more "awful" the gag, 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.

"It creates a common experience at the gathering and I think it's wonderful."