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

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

This describes a humor-evaluation session with a firm that produces products for social events. Its catalogue features festive crackers.

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

"You measure the joke by the volume of groans and the intensity of the groans around the table," she explains.

The key to a great Christmas cracker joke is not the same as a stand-up gag in itself. It is all about the context - in this instance, the shared amusement of the Christmas meal with grandparents, children and potentially friends.

"You want the gag to be something that brings the eight-year-old together with the grandparent," she states.

The Science Behind Communal Amusement

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

"Therefore when you are chuckling with others around the holiday table you are dropping into what's almost certainly a truly ancient mammal play vocalisation," explains a neuroscience expert.

Communal amusement, she says, helps forge and strengthen social bonds between people.

Researchers have discovered that a lack of these interactions can seriously harm mental and physical well-being.

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

Endorphins are the brain's "happy chemicals" and are released both to alleviate stress and pain and in reaction to pleasurable activities, such as chuckling with loved ones over a particularly awful Christmas cracker joke.

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

Which Occurs In the Brain?

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

An awful lot happens in response to humour, it turns out.

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

Testing involves scanning the brains of volunteer subjects and then subjecting them to a database of humorous phrases, paired with either a neutral sound, or pre-recorded chuckles.

"In the scanner we got a really fascinating activation pattern of activation," says the neuroscientist.

A gag activates not just the areas of the mind in charge of hearing and interpreting speech, but also brain regions involved in both planning and initiating motion and those linked to sight and recall.

Combine all of this as a whole, and people listening to a joke have a complex series of brain reactions that support the amusement we experience.

The Infectious Power of Laughter

Scientists discovered that when a funny word is paired with chuckles there is a stronger response in the mind than the same phrase when followed by a neutral sound.

"This activation occurred in parts of the brain that you would use to contort your face into a smile or a laugh," she says.

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

Laughter, says the professor, can be contagious.

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

"People laugh more when you are familiar with people," she says, "and laughter increases further when you like them or care for them."

When it comes to Christmas cracker puns, she explains, the positive effect is more probable to be caused not by the gag itself, but from the response to it.

"The laughter is key. The joke is the dreadful holiday cracker pun, and it's just a pretext to laugh together."

The Quest for the Ideal Festive Pun

Is it possible to discover the ultimate joke?

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

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

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

The ideal festive cracker pun must be short, he explains.

"But they also be poor jokes, puns that make us moan," he continues.

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

"The reason is that if nobody laughs – it's the joke's shortcoming, not yours.

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

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