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

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

We're at a humor-evaluation meeting with a company that produces supplies for social events. Its catalogue includes festive crackers.

The company's founder grins, almost sheepishly at the joke. But the joke has been selected and will appear in future crackers.

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

The key to a great holiday cracker joke is not the identical as a good joke in itself. It is entirely about the setting - in this case, the communal amusement of the Christmas meal with grandparents, kids and possibly friends.

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

The Neuroscience Behind Shared Amusement

Gathering to enjoy communal laughter is not only nothing new, scientists argue, it is likely to be pre-human.

"Therefore when you are chuckling with people at the holiday table you are dropping into what's almost certainly a truly primordial mammal social vocalisation," says a professor.

Communal laughter, she explains, helps make and maintain social bonds between individuals.

Scientists have found that a lack of such interactions can significantly harm mental and physical well-being.

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

Endorphins are the body's "happy chemicals" and are produced both to alleviate stress and pain and in response to pleasurable experiences, such as chuckling with friends over a particularly terrible Christmas cracker joke.

"It's not simply laughing at a foolish joke with a holiday cracker," she states. "You are in fact doing a lot of the truly important task of making, maintaining the connections you have with those you care about."

What Occurs In the Brain?

But what is truly taking place within the mind when we hear a joke?

A tremendous amount occurs in response to comedy, it transpires.

Employing brain scanning technology, a kind of brain scanner which shows which areas of the mind are more active, researchers have been able to map the areas that get more blood.

The research entails scanning the minds of healthy participants and then exposing them to a collection of humorous phrases, paired with either a non-emotional sound, or recorded chuckles.

"In the scanner we got a really interesting pattern of activation," says the professor.

A joke activates not just the areas of the mind in charge of hearing and interpreting speech, but also neural areas associated with both preparation and initiating movement and those linked to sight and recall.

Combine all of this together, and people hearing a pun have a sophisticated set of brain responses that support the laughter we hear.

The Contagious Power of Laughter

Scientists found that when a humorous phrase is combined with laughter there is a stronger reaction in the brain than the same phrase when accompanied by a neutral sound.

"This was in areas of the mind that you would employ to contort your face into a grin or a chuckle," the professor explains.

It means we are not just reacting to humorous jokes, they are reacting to the laughter that accompanies them.

Laughter, says the expert, can be contagious.

So what does this mean for the laughter found around a holiday gathering?

"You laugh harder when you know others," she says, "and laughter increases further when you like them or love them."

When it comes to festive cracker puns, she explains, the feel-good 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 terrible holiday cracker joke, and it's just a reason to laugh as a group."

The Search for the Ideal Festive Pun

Will we ever find the perfect gag?

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

Years ago, a professor set up a scientific search for the world's funniest joke.

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

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

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

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

"This is because if no-one finds it funny – it's the joke's shortcoming, not your own.

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

"It creates a shared moment around the gathering and I think it's wonderful."