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

This one-liner is met by moans that resonate through a warehouse in the capital.

We're at a joke-testing session with a firm that produces products for social events. Its catalogue features festive crackers.

The firm's owner smiles, nearly apologetically at the gag. But the joke has been selected and will feature in upcoming crackers.

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

The key to a good Christmas cracker joke is not the same as a stand-up gag per se. It is entirely about the setting - 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 child in harmony with the grandparent," she states.

The Neuroscience Of Communal Amusement

Gathering to experience shared amusement is not only nothing new, experts argue, it is probably to be older than humanity.

"So when you are laughing with others around the holiday table you are engaging in what's very likely a truly ancient mammalian social vocalisation," explains a professor.

Communal amusement, she says, helps make and maintain social connections between people.

Scientists have found that a absence of such interactions can seriously damage mental and physical health.

"Those you converse with, and laugh with, it leads to increased levels of endorphin release," she continues.

These natural chemicals are the brain's "feel-good compounds" and are released both to reduce stress and pain and in reaction to enjoyable experiences, such as chuckling with loved ones over a truly terrible Christmas cracker joke.

"It's not simply chuckling at a silly pun with a Christmas cracker," she says. "You are in fact performing a lot of the really vital work of building, preserving the connections you have with those you love."

What Occurs Inside the Mind?

But what is actually taking place inside the mind when we hear a gag?

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

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the brain are more active, scientists have been able to map the regions that receive more blood.

Testing involves imaging the brains of healthy subjects and then subjecting them to a collection of humorous words, accompanied by either a neutral sound, or pre-recorded laughter.

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

A gag activates not just the parts of the brain in charge of auditory processing and understanding language, but also neural areas involved in both preparation and initiating movement and those involved in sight and recall.

Put these elements together, and people listening to a joke have a sophisticated set of neural reactions that underpin the laughter we hear.

The Contagious Power of Laughter

Scientists discovered that when a funny phrase is paired with laughter there is a stronger response in the brain than the identical phrase when followed by a neutral sound.

"This was in parts of the brain that you would employ to contort your expression into a smile or a chuckle," the professor says.

It means we are not just responding to humorous jokes, they are responding to the amusement that accompanies them.

Laughter, according to the professor, can be contagious.

So what does this mean for the laughter found around a holiday gathering?

"You laugh harder when you are familiar with others," she notes, "and laughter increases more when you like them or love them."

When it comes to festive cracker jokes, she explains, the feel-good effect is more likely to be triggered not by the gag in itself, but from the reaction to it.

"The laughter is key. The joke is the terrible holiday cracker joke, and it's just a reason to chuckle as a group."

The Search for the Ideal Festive Pun

Will we ever discover the perfect gag?

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

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

More than 40,000 jokes submitted, with scores provided by hundreds of thousands of people around the world, he has a better idea than many as to what succeeds and what does not.

The perfect festive cracker pun needs to be brief, he says.

"They must also be bad jokes, puns that cause us to groan," he continues.

The more "awful" the gag, he states the better.

"This is because if nobody finds it funny – it's the gag's shortcoming, not yours.

"What's interesting about the Christmas cracker puns is that none of us find them humorous.

"That's a shared moment at the table and I think it's lovely."