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

This joke is greeted with moans that echo through a storage facility in London.

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

The company's founder smiles, nearly sheepishly at the joke. But the joke has been selected and will feature in future crackers.

"The success is gauged by the gag by the number of moans and the intensity of the groans at the table," she explains.

The key to a good Christmas cracker joke is not the identical as a good joke per se. It is entirely about the setting - in this case, the communal laughter of the holiday meal with elders, children and possibly friends.

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

The Neuroscience Of Shared Laughter

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

"Therefore when you are chuckling with people at the holiday dinner you are engaging in what's very likely a truly ancient mammal play sound," says a neuroscience expert.

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

Researchers have found that a lack of such social exchanges can seriously damage mental and physical health.

"The people you converse with, and share laughter with, it leads to increased amounts of endorphin uptake," she continues.

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

"You're not just chuckling at a silly pun with a holiday cracker," the expert states. "You are in fact doing a lot of the truly vital task of making, maintaining the social bonds you have with the people you care about."

Which Happens In the Brain?

But what is actually happening within the mind when we listen to a joke?

An awful lot occurs in response to humour, it transpires.

Using brain scanning technology, a type of neural imager which indicates which areas of the brain are working harder, researchers have been able to chart the regions that receive more blood flow.

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

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

A gag stimulates not just the parts of the mind responsible for auditory processing and understanding speech, but also brain areas associated with both preparation and initiating movement and those linked to vision and memory.

Put all of this as a whole, and individuals listening to a joke have a sophisticated series of brain responses that underpin the laughter we experience.

The Infectious Power of Laughter

Scientists found that when a humorous word is combined with laughter there is a greater reaction in the mind than the identical word when followed by a neutral sound.

"This activation occurred in parts of the brain that you would employ to contort your expression into a smile or a laugh," she explains.

It means we are not just responding to humorous jokes, they are responding to the amusement that follows them.

Laughter, says the expert, can be contagious.

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

"People laugh harder when you know others," she notes, "and laughter increases further when you are fond of them or love them."

When it comes to Christmas cracker puns, she explains, 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 Search for the Ideal Festive Pun

Will we ever discover the ultimate gag?

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

In 2001, a psychologist set up a scientific search for the world's most humorous joke.

More than 40,000 gags submitted, with scores lodged by 350,000 participants globally, he has a clearer idea than most as to what works and what does not.

The ideal Christmas cracker pun must be short, he says.

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

The increasingly "terrible" the joke, he says the better.

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

"The fascinating part about the holiday cracker jokes is that not one person considers them humorous.

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