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

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

We're at a humor-evaluation meeting with a company that produces supplies for gatherings. Its catalogue features Christmas crackers.

The firm's owner grins, nearly sheepishly at the gag. But the pun has made the cut and will appear in upcoming crackers.

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

The secret to a good Christmas cracker pun is not the same as a stand-up joke per se. It is all about the setting - in this case, the communal amusement of the Christmas meal with elders, children and possibly neighbours.

"The goal is for the joke to be a thing that brings the eight-year-old together with the 80-year-old," she states.

The Neuroscience Of Shared Amusement

Gathering to enjoy communal laughter is not only ancient, experts say, it is probably to be pre-human.

"So when you are laughing with people at the Christmas dinner you are engaging in what's very likely a truly primordial mammalian social vocalisation," says a neuroscience expert.

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

Scientists have discovered that a lack of such interactions can significantly damage both psychological and bodily health.

"Those you talk to, and laugh with, it results in enhanced amounts of endorphin release," she adds.

Endorphins are the brain's "feel-good compounds" and are produced both to alleviate tension and discomfort and in reaction to enjoyable activities, such as laughing with friends over a particularly terrible festive cracker joke.

"You're not just laughing at a foolish pun 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 care about."

What Happens Inside the Mind?

But what is actually taking place within the brain when we listen to a joke?

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

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which areas of the mind are more active, scientists have been able to map the areas that get more blood.

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

"During the study we got a very fascinating activation pattern of activation," says the neuroscientist.

A joke activates not just the parts of the brain responsible for auditory processing and interpreting language, but also brain regions associated with both preparation and initiating movement and those linked to sight and memory.

Combine all of this together, and individuals hearing a joke have a complex set of neural reactions that underpin the amusement we hear.

The Contagious Power of Chuckles

Scientists found that when a funny phrase is combined with laughter there is a stronger reaction in the brain than the same phrase when accompanied by a non-emotional sound.

"This activation occurred in areas of the brain that you would employ to contort your expression into a smile or a laugh," the professor says.

It indicates people are not just responding to funny words, they are responding to the amusement that accompanies them.

Laughter, says the expert, can be contagious.

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

"You laugh more when you know others," she says, "and you laugh more when you are fond of them or love them."

When it comes to festive cracker jokes, she explains, the feel-good effect is more likely to be triggered not by the gag in itself, but from the response to it.

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

The Quest for the Perfect Festive Pun

Is it possible to find the ultimate joke?

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

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

Over 40,000 jokes submitted, with ratings provided by hundreds of thousands of people around the world, he has a clearer understanding than most as to what succeeds and what fails.

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

"They must also be bad gags, jokes that cause us to groan," he adds.

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

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

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

"That's a common moment at the table and I think it's wonderful."