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

This one-liner is greeted with groans that echo through a storage facility in London.

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

The company's owner grins, almost sheepishly at the gag. But the pun has made the cut and will appear in future crackers.

"You measure the joke by the number of groans and the loudness of the groans around the table," she explains.

The secret to a good Christmas cracker pun is not the same as a good joke in itself. It is all about the context - in this case, the communal amusement of the holiday meal with grandparents, children and potentially friends.

"The goal is for the joke to be something that brings the child in harmony with the 80-year-old," she adds.

The Neuroscience Behind Shared Laughter

Gathering to experience shared amusement is not only nothing new, scientists argue, it is likely to be older than humanity.

"So when you are laughing with others at the holiday dinner you are dropping into what's very likely a truly ancient mammalian play sound," says a neuroscience expert.

Shared laughter, she says, helps forge and strengthen social bonds between individuals.

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

"The people you converse with, and laugh with, it results in enhanced amounts of endorphin uptake," the professor adds.

Endorphins are the body's "happy chemicals" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as laughing with loved ones over a truly terrible festive cracker joke.

"It's not simply laughing at a foolish joke with a holiday cracker," the expert states. "You are in fact performing a lot of the really important work of making, maintaining the connections you have with the people you care about."

Which Happens In the Brain?

But what is actually taking place inside the mind when we listen to a gag?

An awful lot happens in reaction to humour, it turns out.

Using brain scanning technology, a type of brain scanner which shows which parts of the mind are working harder, scientists have been able to map the areas that receive more blood.

The research entails imaging the minds of volunteer participants and then exposing them to a database of humorous phrases, accompanied by either a non-emotional sound, or recorded laughter.

"In the scanner we got a really interesting activation pattern of neural activity," notes the professor.

A gag activates not just the areas of the brain in charge of auditory processing and interpreting language, but also neural areas associated with both planning and initiating movement and those involved in sight and recall.

Put these elements together, and individuals hearing a joke have a sophisticated series of neural reactions that underpin the amusement we hear.

The Infectious Power of Chuckles

Researchers discovered that when a funny phrase is combined with chuckles there is a greater response in the brain than the same word when followed by a non-emotional sound.

"This was in areas of the brain that you would employ to contort your expression into a smile or a chuckle," she explains.

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

Laughter, according to the expert, can be infectious.

So what does this mean for the laughter heard at a holiday table?

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

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

"The laughter is key. The joke is the dreadful holiday cracker pun, and it's just a reason to chuckle as a group."

The Quest for the Ideal Festive Pun

Will we ever discover the ultimate joke?

Likely not, but that has not prevented experts from trying to.

Years ago, a psychologist set up a research project for the world's funniest gag.

Over tens of thousands of jokes submitted, with ratings lodged by hundreds of thousands of participants globally, he has a clearer understanding than many as to what works and what does not.

The perfect Christmas cracker joke needs to be short, he explains.

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

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

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

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

"It creates a shared moment at the table and I believe it's wonderful."