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

This quip is greeted with moans that echo through a warehouse in London.

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

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

"You measure the gag by the volume of moans and the intensity of the groans around the table," the founder says.

The secret to a great holiday cracker pun is not the identical as a good joke in itself. It is entirely about the context - in this instance, the shared laughter of the Christmas meal with grandparents, kids and possibly neighbours.

"You want the joke to be something that brings the child in harmony with the 80-year-old," she states.

The Science Behind Communal Amusement

Gathering to experience shared amusement is not only nothing new, scientists say, it is likely 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 ancient mammal play sound," explains a professor.

Communal laughter, she explains, helps make and maintain social bonds between people.

Researchers have found that a absence of such interactions can significantly harm both psychological and bodily health.

"Those you converse with, and share laughter with, it results in increased amounts of endorphin uptake," she continues.

Endorphins are the body's "feel-good compounds" and are released both to reduce stress and pain and in reaction to enjoyable experiences, such as laughing with loved ones over a particularly terrible festive cracker gag.

"You're not just laughing at a silly joke with a Christmas cracker," she states. "You are actually doing a lot of the truly vital work of making, maintaining the social bonds you have with the people you care about."

Which Occurs In the Mind?

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

A tremendous amount occurs in response to humour, it transpires.

Employing brain scanning technology, a kind of neural imager which indicates which parts of the brain are more active, scientists have been able to map the regions that receive more blood flow.

Testing entails imaging the minds of volunteer participants and then exposing them to a database of funny words, paired with either a non-emotional sound, or pre-recorded laughter.

"During the study we observed a really interesting activation pattern of activation," says the neuroscientist.

A joke activates not just the parts of the mind in charge of auditory processing and interpreting speech, but also neural areas involved in both planning and initiating movement and those involved in sight and memory.

Put all of this as a whole, and people hearing a joke have a sophisticated series of neural reactions that underpin the laughter we hear.

The Infectious Power of Laughter

Researchers discovered that when a humorous word is paired with laughter there is a stronger response in the mind than the identical phrase when accompanied by a non-emotional sound.

"This was in parts of the mind that you would use to move your face into a smile or a laugh," she says.

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

Amusement, according to the professor, can be infectious.

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

"People laugh harder when you are familiar with people," she notes, "and laughter increases more when you like them or care for them."

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

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

The Quest for the Ideal Cracker Joke

Will we ever discover the perfect gag?

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

In 2001, a professor set up a research search for the planet's most humorous gag.

More than tens of thousands of jokes later, with ratings lodged by 350,000 participants around the world, he has a clearer idea than most as to what works and what fails.

The perfect Christmas cracker joke needs to be brief, he says.

"But they also need to be poor gags, jokes that make us moan," he adds.

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

"The reason is that if nobody finds it funny – it's the gag's shortcoming, not yours.

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

"That's a common experience around the table and I believe it's lovely."