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"How much did Father Christmas's sled cost? Zero, it was on the house."

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

We're at a humor-evaluation session with a firm that produces products for social events. Its repertoire includes festive crackers.

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

"The success is gauged by the joke by the volume of moans and the loudness of the groans around the table," the founder says.

The key to a good holiday cracker joke is not the identical as a stand-up joke in itself. It is all about the context - in this case, the communal laughter of the Christmas dinner table with elders, kids and potentially friends.

"You want the joke to be a thing that unites the eight-year-old in harmony with the 80-year-old," she states.

The Science Of Shared Laughter

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

"So when you are chuckling with people at the Christmas table you are engaging in what's almost certainly a really ancient mammalian play sound," explains a neuroscience expert.

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

Researchers have discovered that a absence of such interactions can seriously harm mental and physical well-being.

"Those you talk to, and share laughter with, it results in enhanced levels of endorphin release," the professor adds.

Endorphins are the brain's "happy chemicals" and are released both to alleviate stress and pain and in response to enjoyable activities, such as laughing with loved ones over a truly terrible Christmas cracker gag.

"It's not simply laughing at a silly joke with a holiday cracker," the expert states. "You are actually doing a lot of the really important task of making, maintaining the social bonds you have with those you love."

What Happens Inside the Brain?

But what is truly happening inside the brain when we listen to a gag?

An awful lot occurs in reaction to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the mind are working harder, scientists have been able to chart the areas that get more blood.

The research involves imaging the brains of healthy subjects and then exposing them to a collection of humorous words, accompanied by either a neutral sound, or recorded laughter.

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

A joke activates not just the areas of the brain in charge of auditory processing and understanding language, but also brain areas associated with both preparation and starting movement and those involved in sight and memory.

Put these elements together, and people hearing a pun have a sophisticated set of brain responses that underpin the laughter we hear.

The Infectious Nature of Laughter

Researchers discovered that when a funny phrase is combined with chuckles there is a stronger reaction in the mind than the identical phrase when accompanied by a neutral sound.

"This was in parts of the brain that you would use to move your expression into a smile or a chuckle," the professor explains.

It means we are not just reacting to funny words, they are responding to the laughter that accompanies them.

Laughter, says the expert, can be infectious.

So what does this imply for the laughter heard around a holiday gathering?

"People laugh more when you are familiar with people," she says, "and you laugh further when you like them or care for them."

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

"The laughter is key. The gag is the dreadful Christmas cracker pun, and it's just a reason to laugh together."

The Quest for the Perfect Cracker Joke

Will we ever discover the perfect joke?

Probably not, but that has not stopped researchers from trying to.

Years ago, a psychologist set up a research search for the world's most humorous joke.

Over tens of thousands of jokes later, with scores lodged by 350,000 people around the world, he has a clearer understanding than most as to what works and what fails.

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

"They must also be poor jokes, jokes that make us moan," he adds.

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

"This is because if no-one finds it funny – it's the gag's shortcoming, not your own.

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

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