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"How much did Father Christmas's sled cost? Nothing, it was on the house."

This joke is met by moans that echo through a warehouse in London.

This describes a joke-testing meeting with a company that makes products for gatherings. Its catalogue features Christmas crackers.

The firm's owner grins, almost sheepishly at the joke. But the pun has made the cut and will appear in upcoming crackers.

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

The key to a great Christmas cracker joke is not the same as a good gag in itself. It is all about the context - in this case, the communal laughter of the Christmas dinner table with elders, kids and possibly friends.

"The goal is for the joke to be a thing that unites the child together with the 80-year-old," she adds.

The Neuroscience Behind Communal Laughter

Gathering to experience shared amusement is not only ancient, scientists argue, it is likely to be pre-human.

"So when you are laughing with others at the Christmas dinner you are engaging in what's very likely a really ancient mammalian social sound," explains a professor.

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

Researchers have found that a lack of such interactions can seriously harm both psychological and bodily health.

"The people you converse with, and laugh with, it results in increased levels of 'happy chemical' uptake," the professor continues.

Endorphins are the body's "feel-good compounds" and are produced both to alleviate stress and pain and in reaction to pleasurable experiences, such as chuckling with loved ones over a particularly awful festive cracker gag.

"You're not just laughing at a silly joke with a holiday cracker," the expert says. "You are actually doing a lot of the really important work of making, maintaining the connections you have with those you care about."

Which Occurs Inside the Brain?

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

A tremendous amount happens in reaction to comedy, it turns out.

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which areas of the brain are working harder, scientists have been able to map the regions that get more blood flow.

The research entails imaging the brains of healthy participants and then subjecting them to a database of funny phrases, accompanied by either a non-emotional sound, or recorded chuckles.

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

A joke activates not just the areas of the mind responsible for auditory processing and understanding language, but also neural areas associated with both preparation and starting motion and those involved in vision and memory.

Combine these elements as a whole, and individuals hearing a joke have a sophisticated set of neural responses that underpin the amusement we experience.

The Contagious Nature of Laughter

Scientists discovered 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 activation occurred in areas of the mind that you would use to contort your expression into a smile or a chuckle," she explains.

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

Amusement, according to the professor, can be infectious.

So what does this mean for the chuckles heard at a holiday gathering?

"People laugh harder when you are familiar with people," she says, "and you laugh more when you like them or love them."

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

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

The Quest for the Perfect Festive Pun

Will we ever discover the perfect joke?

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

In 2001, a psychologist established a research search for the planet's funniest gag.

Over 40,000 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 ideal festive cracker pun must be short, he explains.

"But they also need to be poor jokes, jokes that make us moan," he adds.

The increasingly "terrible" the joke, he states the better.

"This is because if no-one laughs – it's the gag's shortcoming, not yours.

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

"That's a common moment at the table and I think it's wonderful."