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"How much did Santa's sleigh cost? Zero, it was on the house."

This quip is greeted with groans that echo through a storage facility in London.

We're at a humor-evaluation session with a firm that makes products for gatherings. Its catalogue includes festive crackers.

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

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

The secret to a good holiday cracker joke is not the same as a good joke in itself. It is all about the context - in this case, the communal amusement of the Christmas dinner table with elders, children and possibly friends.

"The goal is for the joke to be something that brings the child together with the grandparent," she adds.

The Neuroscience Behind Communal Laughter

Coming together to experience shared laughter is not only nothing new, scientists argue, it is probably to be older than humanity.

"Therefore when you are laughing with others at the holiday dinner you are dropping into what's almost certainly a truly ancient mammalian play vocalisation," explains a neuroscience expert.

Communal laughter, she says, helps forge and strengthen social connections between individuals.

Scientists have found that a lack of these social exchanges can significantly damage mental and physical well-being.

"Those you talk to, and laugh with, it leads to enhanced amounts of endorphin uptake," the professor adds.

Endorphins are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in reaction to pleasurable activities, such as chuckling with friends over a particularly awful festive cracker joke.

"It's not simply laughing at a silly pun with a holiday cracker," the expert states. "You are actually doing a lot of the truly important task of building, preserving the connections you have with those you love."

Which Occurs In the Brain?

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

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

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

The research entails imaging the minds of volunteer subjects and then subjecting them to a database of humorous phrases, accompanied by either a non-emotional sound, or recorded chuckles.

"During the study we got a really fascinating pattern of neural activity," notes the neuroscientist.

A gag stimulates not just the parts of the brain in charge of hearing and understanding language, but also brain regions involved in both preparation and starting movement and those linked to vision and recall.

Combine all of this together, and people hearing a joke have a complex series of neural responses that support the amusement we experience.

The Contagious Nature of Laughter

Scientists discovered that when a humorous phrase is combined with laughter there is a stronger response in the brain than the identical word when followed by a non-emotional sound.

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

It means people are not just reacting to funny jokes, they are reacting to the amusement that follows them.

Amusement, according to the expert, can be infectious.

So what does this mean for the chuckles found around a Christmas gathering?

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

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

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

The Quest for the Ideal Cracker Joke

Will we ever discover the ultimate joke?

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

In 2001, a psychologist set up a scientific search for the world's funniest gag.

More than tens of thousands of jokes submitted, with scores provided by hundreds of thousands of people globally, he has a clearer understanding than most as to what works and what does not.

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

"But they also be bad gags, jokes that cause us to groan," he adds.

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

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

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

"It creates a common moment around the gathering and I believe it's lovely."