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"What was the price did Santa's sled cost? Nothing, it was on the house."

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

This describes a humor-evaluation session with a company that makes supplies for social events. Its catalogue includes festive crackers.

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

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

The secret to a great Christmas cracker pun is not the same 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 potentially friends.

"The goal is for the gag to be something that brings the child together with the 80-year-old," she states.

The Neuroscience Behind Communal Laughter

Coming together to experience communal laughter is not only ancient, scientists argue, it is likely to be pre-human.

"So when you are laughing with people at the holiday dinner you are engaging in what's very likely a really primordial mammal social vocalisation," explains a professor.

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

Scientists have found that a lack of such interactions can significantly harm mental and physical well-being.

"Those you talk to, and laugh with, it leads to enhanced amounts of 'happy chemical' release," the professor adds.

These natural chemicals are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in response to enjoyable experiences, such as chuckling with friends over a truly awful festive cracker gag.

"It's not simply laughing at a foolish pun with a Christmas cracker," she states. "You are actually performing a lot of the really important task of making, maintaining the connections you have with the people you love."

Which Occurs In the Brain?

But what is actually happening inside the mind when we hear 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 mind are working harder, researchers have been able to chart the regions that get more blood.

The research entails imaging the minds of volunteer participants and then subjecting them to a database of funny phrases, accompanied by either a neutral sound, or recorded laughter.

"In the scanner we observed a very interesting activation pattern of activation," notes the professor.

A gag activates not just the areas of the mind responsible for hearing and interpreting speech, but also brain regions involved in both planning and starting motion and those linked to vision and recall.

Combine all of this together, and people listening to a pun have a complex series of brain reactions that underpin the laughter we hear.

The Infectious Nature of Laughter

Researchers discovered that when a funny phrase is combined with laughter there is a stronger response in the brain than the same phrase when followed by a neutral sound.

"This activation occurred in parts of the brain that you would employ to move your expression into a grin or a laugh," she says.

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

Amusement, according to the professor, can be contagious.

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

"You laugh harder when you are familiar with others," she notes, "and laughter increases further when you are fond of them or care for them."

When it comes to festive cracker puns, she says, the positive factor is more probable to be triggered not by the gag in itself, but from the response to it.

"The laughter is key. The gag is the terrible Christmas cracker joke, and it's just a pretext to laugh together."

The Search for the Ideal Festive Pun

Is it possible to find the perfect gag?

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

Years ago, a psychologist set up a scientific project for the world's funniest joke.

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

The ideal festive cracker joke needs to be short, he explains.

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

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

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

"What's interesting about the holiday cracker puns is that not one person find them funny.

"It creates a common experience around the gathering and I believe it's lovely."