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

This one-liner is greeted with moans that echo through a warehouse in London.

This describes a joke-testing session with a firm that makes supplies for social events. Its catalogue includes festive crackers.

The firm's founder grins, nearly apologetically at the gag. But the joke has made the cut and will feature in future crackers.

"You measure the joke by the volume of moans and the intensity of the groans around the table," she explains.

The key to a great Christmas cracker joke is not the same as a stand-up gag in itself. It is entirely about the setting - in this case, the communal amusement of the Christmas dinner table with elders, children and possibly neighbours.

"You want the joke to be a thing that brings the eight-year-old in harmony with the grandparent," she states.

The Neuroscience Behind Communal Amusement

Coming together to experience communal laughter is not only ancient, experts say, it is likely to be older than humanity.

"So when you are chuckling with people at the holiday table you are engaging in what's almost certainly a really primordial mammal social sound," says a professor.

Communal amusement, she explains, helps forge and strengthen social bonds between individuals.

Researchers have discovered that a absence of these social exchanges can significantly damage mental and physical well-being.

"The people you talk to, and share laughter with, it leads to enhanced amounts of 'happy chemical' uptake," the professor continues.

Endorphins are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in response to enjoyable activities, such as chuckling with friends over a truly terrible festive cracker joke.

"It's not simply laughing at a silly joke with a Christmas cracker," the expert says. "You are actually performing a lot of the truly vital work of making, maintaining the social bonds you have with those you care about."

Which Happens Inside the Brain?

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

A tremendous amount occurs in response to humour, it turns out.

Using brain scanning technology, a kind of neural imager which shows which areas of the brain are working harder, scientists have been able to chart the areas that receive more blood.

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

"During the study we got a really fascinating activation pattern of neural activity," notes the neuroscientist.

A joke stimulates not just the areas of the mind in charge of auditory processing and understanding language, but also neural regions associated with both planning and starting motion and those involved in sight and recall.

Put these elements together, and people hearing a pun have a complex set of neural reactions that support the laughter we hear.

The Contagious Power of Chuckles

Scientists discovered that when a humorous word is combined with laughter there is a stronger reaction in the brain than the identical phrase when accompanied by a neutral sound.

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

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

Amusement, says the expert, can be infectious.

So what does this imply for the laughter found at a Christmas gathering?

"You laugh harder when you know others," she says, "and you laugh more when you like them or care for them."

When it comes to Christmas cracker jokes, she says, the feel-good factor is more probable to be caused not by the joke itself, but from the response to it.

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

The Search for the Ideal Cracker Joke

Is it possible to discover the perfect joke?

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

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

More than 40,000 gags submitted, with scores provided by hundreds of thousands of people around the world, he has a better idea than most as to what works and what fails.

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

"But they also be poor gags, puns that make us moan," he adds.

The increasingly "terrible" the gag, he states the more effective.

"This is because if no-one laughs – it's the gag's fault, not your own.

"The fascinating part about the holiday cracker jokes is that none of us find them funny.

"It creates a shared moment at the gathering and I think it's lovely."