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

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

We're at a joke-testing meeting with a company that produces products for gatherings. Its catalogue features Christmas crackers.

The company's founder grins, almost apologetically at the gag. But the pun has made the cut and will feature in future crackers.

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

The secret to a great Christmas cracker pun is not the same as a stand-up gag per se. It is entirely about the context - in this case, the shared laughter of the Christmas dinner table with grandparents, children and possibly neighbours.

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

The Science Behind Shared Laughter

Coming together to enjoy communal amusement is not only ancient, scientists say, it is likely to be pre-human.

"Therefore when you are laughing with people around the holiday table you are engaging in what's very likely a really primordial mammalian play vocalisation," explains a professor.

Communal amusement, she explains, helps forge and strengthen social connections between individuals.

Researchers have discovered that a absence of such social exchanges can seriously harm both psychological and bodily well-being.

"Those you talk to, and share laughter with, it results in increased amounts of endorphin release," the professor adds.

Endorphins are the brain's "feel-good compounds" and are released both to alleviate stress and pain and in response to enjoyable experiences, such as laughing with loved ones over a particularly terrible Christmas cracker joke.

"It's not simply chuckling at a silly joke with a holiday cracker," she says. "You are actually performing a lot of the really vital task of making, maintaining the connections you have with those you care about."

Which Occurs In the Brain?

But what is truly happening inside the brain when we listen to a joke?

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

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which parts of the mind are more active, researchers have been able to map the regions that get more blood flow.

Testing involves imaging the minds of healthy participants and then exposing them to a collection of humorous words, accompanied by either a non-emotional sound, or recorded chuckles.

"In the scanner we observed a very fascinating activation pattern of activation," says the neuroscientist.

A gag stimulates not just the parts of the mind responsible for hearing and interpreting language, but also brain regions associated with both preparation and starting movement and those linked to sight and recall.

Combine all of this as a whole, and individuals hearing a pun have a complex set of neural reactions that support the amusement we hear.

The Infectious Power of Laughter

Scientists discovered that when a humorous word is combined with chuckles there is a stronger response in the brain than the same word when accompanied by a neutral sound.

"This activation occurred in areas of the brain that you would employ to move your expression into a smile or a laugh," the professor explains.

It indicates we are not just responding to funny words, they are reacting to the amusement that accompanies them.

Laughter, says the professor, can be infectious.

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

"People laugh more when you know people," she says, "and laughter increases further when you like them or love them."

When it comes to festive cracker puns, she explains, the positive factor is more likely to be triggered not by the gag in itself, but from the reaction to it.

"It's the laughter. The joke is the terrible holiday cracker joke, and it's just a pretext to laugh as a group."

The Search for the Ideal Festive Pun

Is it possible to discover the perfect joke?

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

In 2001, a professor established a scientific search for the planet's most humorous joke.

Over tens of thousands of jokes later, with scores provided by hundreds of thousands of people around the world, he has a better understanding than most as to what works and what fails.

The perfect Christmas cracker pun needs to be brief, he says.

"But they also be bad gags, puns that make us moan," he adds.

The increasingly "awful" the gag, he says the better.

"The reason is that if nobody finds it funny – it's the gag's fault, not yours.

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

"It creates a common experience at the table and I believe it's wonderful."