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

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

We're at a joke-testing meeting with a company that produces supplies for social events. Its repertoire features Christmas crackers.

The firm's owner grins, almost apologetically at the gag. But the pun has made the cut and will feature in upcoming crackers.

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

The key to a great holiday cracker pun is not the same as a stand-up joke in itself. It is entirely about the context - in this case, the communal laughter of the holiday dinner table with grandparents, kids and potentially friends.

"You want the gag to be a thing that brings the eight-year-old together with the 80-year-old," she adds.

The Neuroscience Behind Communal Amusement

Coming together to experience shared laughter is not only ancient, experts argue, it is probably to be older than humanity.

"Therefore when you are laughing with people at the holiday table you are dropping into what's almost certainly a really ancient mammalian play sound," explains a neuroscience expert.

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

Researchers have discovered that a absence of such interactions can significantly damage both psychological and bodily health.

"The people you talk to, and share laughter with, it results in enhanced amounts of 'happy chemical' uptake," she adds.

Endorphins are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in response to enjoyable experiences, such as laughing with friends over a particularly terrible Christmas cracker gag.

"You're not just laughing at a silly pun with a holiday cracker," the expert says. "You are actually performing a lot of the really vital task of making, maintaining the social bonds you have with the people you care about."

What Happens In the Brain?

But what is actually taking place inside the mind when we listen to a joke?

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

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

The research entails imaging the minds of healthy subjects and then exposing them to a database of humorous words, paired with either a non-emotional sound, or recorded laughter.

"In the scanner we got a very interesting pattern of activation," says the neuroscientist.

A gag activates not just the areas of the brain in charge of auditory processing and understanding speech, but also brain regions involved in both planning and starting movement and those involved in vision and recall.

Combine all of this as a whole, and individuals listening to a joke have a sophisticated series of neural responses that support the amusement we experience.

The Infectious Power of Laughter

Scientists found that when a humorous phrase is combined with laughter there is a stronger response in the mind than the identical phrase when followed by a neutral sound.

"This activation occurred in parts of the mind that you would employ to contort your face into a smile or a laugh," the professor explains.

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

Amusement, according to the expert, can be infectious.

So what does this mean for the chuckles heard at a Christmas table?

"You laugh more when you know people," she says, "and you laugh further when you are fond of them or care for them."

When it comes to Christmas cracker puns, she says, the feel-good effect is more likely to be caused not by the gag itself, but from the response to it.

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

The Search for the Perfect Festive Pun

Will we ever discover the ultimate joke?

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

Years ago, a psychologist established a scientific search for the world's most humorous gag.

More than 40,000 gags submitted, with scores lodged by hundreds of thousands of people around the world, he has a better idea than most as to what works and what fails.

The perfect Christmas cracker joke must be brief, he says.

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

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

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

"The fascinating part about the holiday cracker puns is that none of us considers them funny.

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