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

This one-liner is met by groans that echo through a storage facility in the capital.

We're at a joke-testing session with a company that makes products for gatherings. Its catalogue includes festive crackers.

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

"You measure the joke by the number of moans and the intensity of the groans around the table," she says.

The secret to a good holiday cracker pun is not the identical as a good gag per se. It is entirely about the context - in this case, the communal amusement of the Christmas meal with elders, kids and possibly friends.

"The goal is for the gag to be a thing that brings the eight-year-old together with the 80-year-old," she states.

The Science Of Communal Amusement

Coming together to enjoy shared laughter is not only ancient, experts say, it is likely to be older than humanity.

"Therefore when you are laughing with others around the Christmas table you are dropping into what's almost certainly a really primordial mammal social vocalisation," says a professor.

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

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

"The people you converse with, and share laughter with, it results in enhanced amounts of 'happy chemical' release," she continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to enjoyable activities, such as laughing with loved ones over a particularly terrible festive cracker joke.

"You're not just laughing at a foolish pun with a holiday cracker," the expert says. "You are actually doing a lot of the really vital work of building, preserving the connections you have with the people you love."

What Happens In the Mind?

But what is truly taking place within the mind when we hear a gag?

An awful lot occurs in response to comedy, it turns out.

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which areas of the mind are working harder, scientists have been able to chart the areas that receive more blood.

The research involves scanning the minds of volunteer participants and then exposing them to a database of funny words, paired with either a non-emotional sound, or recorded chuckles.

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

A joke stimulates not just the parts of the brain responsible for hearing and understanding language, but also neural regions associated with both preparation and initiating movement and those involved in vision and memory.

Put these elements as a whole, and individuals hearing a joke have a sophisticated set of neural responses that underpin the amusement we experience.

The Contagious Nature of Laughter

Researchers found that when a funny word is combined with laughter there is a stronger reaction in the mind than the same word when accompanied by a non-emotional sound.

"This activation occurred in parts of the mind that you would use to move your face into a smile or a laugh," the professor says.

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

Laughter, says the professor, can be contagious.

So what does this imply for the laughter heard around a holiday gathering?

"You laugh more when you know others," she notes, "and laughter increases more when you are fond of them or love them."

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

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

The Search for the Perfect Festive Pun

Will we ever discover the perfect gag?

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

In 2001, a professor set up a research project for the planet's most humorous joke.

More than tens of thousands of jokes submitted, with ratings lodged by 350,000 participants globally, he has a clearer understanding than many as to what succeeds and what does not.

The ideal Christmas cracker joke must be short, he explains.

"But they also be poor jokes, puns that make us groan," he adds.

The more "terrible" the gag, he states the more effective.

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

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

"It creates a shared moment around the gathering and I think it's wonderful."