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

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

This describes a humor-evaluation meeting with a company that produces supplies for social events. Its repertoire includes festive crackers.

The firm's owner grins, almost sheepishly at the joke. But the pun has been selected and will feature in upcoming crackers.

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

The secret to a great Christmas cracker joke is not the same as a stand-up joke per se. It is entirely about the setting - in this case, the shared amusement of the Christmas meal with grandparents, children and possibly friends.

"You want the joke to be something that unites the child together with the grandparent," she states.

The Neuroscience Behind Communal Amusement

Coming together to experience communal amusement is not only nothing new, experts say, it is likely to be pre-human.

"Therefore when you are laughing with others at the holiday dinner you are engaging in what's almost certainly a truly ancient mammalian play sound," explains a neuroscience expert.

Shared laughter, she explains, aids in forge and strengthen social connections between individuals.

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

"Those you talk to, and laugh with, it leads to increased levels of endorphin release," the professor adds.

These natural chemicals are the body's "feel-good compounds" and are produced both to alleviate tension and discomfort and in response to enjoyable experiences, such as laughing with loved ones over a particularly awful festive cracker gag.

"You're not just chuckling at a silly joke with a holiday cracker," she states. "You are in fact doing a lot of the really important work of making, maintaining the social bonds you have with the people you love."

What Occurs Inside the Brain?

But what is truly happening inside the brain when we hear a gag?

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

Employing brain scanning technology, a type of neural imager which indicates which parts of the brain are more active, researchers have been able to map the regions that get more blood.

Testing entails scanning the minds of healthy subjects and then exposing them to a collection of humorous words, paired with either a non-emotional sound, or recorded chuckles.

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

A joke stimulates not just the areas of the brain in charge of hearing and understanding language, but also neural areas associated with both preparation and starting motion and those linked to vision and recall.

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

The Contagious Power of Laughter

Scientists found that when a funny phrase is combined with chuckles there is a greater reaction in the mind than the same word when followed by a neutral sound.

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

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

Amusement, says the professor, can be contagious.

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

"You laugh more when you are familiar with people," she says, "and laughter increases more when you like them or love them."

When it comes to festive cracker jokes, she explains, the feel-good factor is more likely to be triggered not by the joke itself, but from the reaction to it.

"It's the laughter. The gag is the terrible holiday cracker pun, and it's just a reason to chuckle together."

The Search for the Perfect Cracker Joke

Will we ever discover the perfect joke?

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

In 2001, a professor set up a research search for the planet's funniest joke.

More than tens of thousands of jokes later, with scores lodged by hundreds of thousands of participants around the world, he has a better understanding than many as to what works and what does not.

The ideal Christmas cracker pun must be brief, he explains.

"They must also be bad jokes, puns that cause us to moan," he continues.

The increasingly "terrible" the gag, he states the more effective.

"The reason is that if no-one finds it funny – it's the gag's fault, not your own.

"The fascinating part about the Christmas cracker jokes is that none of us considers them humorous.

"It creates a shared moment at the table and I think it's lovely."