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

This quip is greeted with groans that resonate through a storage facility in the capital.

This describes a joke-testing meeting with a firm that produces products for gatherings. Its catalogue includes Christmas crackers.

The firm's owner smiles, almost apologetically at the joke. But the joke has been selected and will appear in future crackers.

"The success is gauged by the joke by the volume of groans and the intensity of the groans around the table," she says.

The secret to a great holiday cracker pun is not the identical as a good joke in itself. It is entirely about the setting - in this case, the communal amusement of the Christmas dinner table with grandparents, children and potentially friends.

"The goal is for the joke to be a thing that unites the eight-year-old in harmony with the 80-year-old," she states.

The Science Behind Communal Amusement

Gathering to enjoy shared laughter is not only ancient, scientists say, it is likely to be pre-human.

"So when you are chuckling with people at the holiday dinner you are dropping into what's almost certainly a truly primordial mammalian play sound," explains a professor.

Shared laughter, she says, helps forge and strengthen social connections between people.

Scientists have found that a lack of such interactions can significantly damage mental and physical health.

"The people you converse with, and laugh with, it leads to increased levels of 'happy chemical' release," the professor adds.

Endorphins are the brain's "happy chemicals" and are produced both to alleviate tension and discomfort and in response to enjoyable experiences, such as laughing with friends over a truly awful festive cracker joke.

"You're not just chuckling at a foolish joke with a Christmas cracker," she says. "You are in fact performing a lot of the truly important work of building, preserving the connections you have with the people you care about."

What Happens Inside the Mind?

But what is truly taking place inside the brain when we listen to a gag?

An awful lot occurs in response to comedy, it transpires.

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

Testing entails scanning the brains of volunteer subjects and then subjecting them to a database of humorous words, paired with either a non-emotional sound, or pre-recorded laughter.

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

A joke stimulates not just the parts of the brain responsible for auditory processing and interpreting language, but also neural regions associated with both preparation and starting motion and those involved in sight and memory.

Put these elements together, and people hearing a joke have a complex set of brain responses that support the amusement we experience.

The Contagious Power of Chuckles

Researchers discovered that when a humorous word is paired with laughter there is a greater response in the brain than the same phrase when followed by a neutral sound.

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

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

Laughter, says the professor, can be infectious.

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

"You laugh harder when you are familiar with people," she says, "and you laugh more when you like them or care for them."

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

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the perfect gag?

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

Years ago, a psychologist established a research search for the world's most humorous joke.

More than tens of thousands of jokes later, with scores lodged by 350,000 people around the world, he has a better understanding than most as to what succeeds and what does not.

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

"They must also be bad jokes, jokes that cause us to moan," he adds.

The more "awful" the gag, he says the more effective.

"The reason is that if nobody laughs – it's the gag's shortcoming, not your own.

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

"That's a common moment around the gathering and I think it's wonderful."