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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 warehouse in the capital.

This describes a humor-evaluation session with a firm that makes products for gatherings. Its catalogue includes Christmas crackers.

The firm's founder grins, almost sheepishly at the gag. But the pun has been selected and will feature in future crackers.

"The success is gauged by the gag by the volume of moans and the loudness of the groans around the table," she 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 case, the shared amusement of the holiday meal with elders, children and potentially friends.

"You want the joke to be something that brings the child in harmony with the grandparent," she adds.

The Science Of Communal Laughter

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

"So when you are laughing with others around the Christmas table you are dropping into what's almost certainly a truly ancient mammalian play vocalisation," explains a neuroscience expert.

Shared amusement, she explains, helps make and maintain social bonds between people.

Scientists have found that a absence of such social exchanges can seriously damage both psychological and bodily health.

"The people you talk to, and laugh with, it leads to enhanced amounts of 'happy chemical' release," the professor adds.

Endorphins are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in response to pleasurable experiences, such as laughing with loved ones over a particularly terrible Christmas cracker gag.

"It's not simply chuckling at a foolish pun with a holiday cracker," the expert states. "You are actually performing a lot of the really important work of making, maintaining the connections you have with the people you care about."

Which Happens In the Brain?

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

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

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

The research involves scanning the minds of healthy subjects and then subjecting them to a database of humorous phrases, paired with either a non-emotional sound, or pre-recorded laughter.

"In the scanner we observed a really fascinating pattern of activation," says the professor.

A joke activates not just the areas of the mind responsible for auditory processing and understanding language, but also brain regions involved in both preparation and initiating movement and those linked to sight and memory.

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

The Infectious Nature of Laughter

Scientists found that when a funny word is combined with chuckles there is a greater response in the brain than the same phrase when followed by a non-emotional sound.

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

It indicates we are not just responding to humorous words, they are reacting to the laughter that accompanies them.

Laughter, according to the expert, can be infectious.

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

"You laugh harder when you know people," she says, "and you laugh further when you like them or care for them."

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

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

The Search for the Ideal Festive Pun

Is it possible to find the perfect gag?

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

Years ago, a psychologist set up a scientific project for the world's funniest joke.

Over tens of thousands of gags submitted, with ratings provided by hundreds of thousands of participants around the world, he has a clearer understanding than most as to what succeeds and what fails.

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

"They must also need to be poor gags, puns that cause us to groan," he continues.

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

"This is because if no-one laughs – it's the gag's fault, not your own.

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

"It creates a common moment at the table and I believe it's lovely."