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

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

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

The company's founder smiles, almost sheepishly at the gag. But the pun has been selected and will feature in upcoming crackers.

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

The key to a great Christmas cracker pun is not the same as a stand-up gag in itself. It is entirely about the setting - in this instance, the communal amusement of the Christmas meal with grandparents, children and possibly friends.

"The goal is for the joke to be something that unites the eight-year-old in harmony with the grandparent," she states.

The Science Behind Shared Laughter

Gathering to enjoy communal laughter is not only ancient, scientists say, it is probably to be older than humanity.

"So when you are chuckling with others at the Christmas dinner you are dropping into what's almost certainly a really primordial mammalian play sound," explains a professor.

Communal amusement, she says, helps make and maintain social bonds between individuals.

Researchers have found that a lack of these social exchanges can significantly harm mental and physical health.

"Those you converse with, and share laughter with, it leads to increased amounts of 'happy chemical' uptake," she adds.

Endorphins are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in reaction to pleasurable activities, such as chuckling with loved ones over a truly awful Christmas cracker joke.

"It's not simply chuckling at a foolish pun with a holiday cracker," she says. "You are in fact doing a lot of the really vital work of building, preserving the connections you have with the people you care about."

Which Occurs In the Mind?

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

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

Using brain scanning technology, a type of neural imager which shows which areas of the brain are more active, researchers have been able to map the areas that get more blood flow.

The research entails imaging the brains of volunteer participants and then subjecting them to a collection of humorous words, accompanied by either a neutral sound, or pre-recorded laughter.

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

A gag stimulates not just the areas of the brain in charge of auditory processing and understanding language, but also neural areas involved in both preparation and starting movement and those involved in vision and memory.

Put these elements together, and individuals listening to a joke have a complex set of neural reactions that underpin the amusement we hear.

The Contagious Power of Chuckles

Researchers found that when a humorous phrase is paired with laughter there is a greater reaction in the brain than the same phrase when followed by a neutral sound.

"This was in parts of the mind that you would use to move your expression into a grin or a chuckle," the professor explains.

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

Amusement, according to the professor, can be contagious.

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

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

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

"It's the laughter. The joke is the dreadful holiday cracker pun, and it's just a pretext to chuckle together."

The Search for the Ideal Festive Pun

Is it possible to find the perfect joke?

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

Years ago, a professor established a research search for the world's funniest joke.

More than 40,000 gags later, with ratings lodged by hundreds of thousands of participants around the world, he has a clearer idea than many as to what works and what does not.

The ideal festive cracker pun must be short, he explains.

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

The increasingly "awful" the joke, he says the more effective.

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

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

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