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"What was the price did Father Christmas's sled cost? Nothing, it was on the house."

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

This describes a humor-evaluation session with a company that makes products for social events. Its repertoire includes Christmas crackers.

The firm's founder smiles, nearly sheepishly at the joke. But the pun has been selected and will appear in upcoming crackers.

"You measure the gag by the volume of moans and the intensity of the groans around the table," the founder says.

The secret to a good holiday cracker joke is not the same as a stand-up gag per se. It is entirely about the context - in this instance, the communal amusement of the holiday dinner table with elders, children and possibly friends.

"You want the gag to be a thing that unites the eight-year-old together with the grandparent," she adds.

The Neuroscience Of Shared Amusement

Coming together to experience communal amusement is not only nothing new, scientists say, it is probably to be older than humanity.

"So when you are laughing with people around the holiday dinner you are engaging in what's almost certainly a truly ancient mammal social sound," says a neuroscience expert.

Shared laughter, she says, helps make and maintain social connections between people.

Researchers have discovered that a absence of such social exchanges can significantly damage both psychological and bodily health.

"Those you talk to, and share laughter with, it leads to increased amounts of 'happy chemical' release," she adds.

Endorphins are the body's "feel-good compounds" and are produced both to reduce tension and discomfort and in reaction to enjoyable experiences, such as laughing with loved ones over a truly awful festive cracker joke.

"You're not just chuckling at a silly pun with a holiday cracker," the expert states. "You are in fact performing a lot of the truly vital work of making, maintaining the connections you have with the people you love."

What Happens Inside the Brain?

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

A tremendous amount happens in response to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which areas of the brain are working harder, scientists have been able to chart the areas that get more blood flow.

Testing entails imaging the brains of healthy subjects and then subjecting them to a database of humorous phrases, accompanied by either a neutral sound, or pre-recorded chuckles.

"In the scanner we got a really fascinating activation pattern of neural activity," notes the neuroscientist.

A gag activates not just the areas of the brain in charge of hearing and understanding language, but also brain areas involved in both preparation and initiating movement and those involved in vision and recall.

Combine these elements together, and people listening to a joke have a sophisticated series of neural responses that support the laughter we experience.

The Infectious Nature of Chuckles

Researchers discovered that when a humorous word is combined with chuckles there is a greater reaction in the brain than the same word when followed by a non-emotional sound.

"This was in areas of the brain that you would employ to contort your face into a grin or a laugh," she says.

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

Amusement, according to the expert, can be contagious.

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

"You laugh more when you are familiar with people," she says, "and you laugh further when you are fond of them or love them."

When it comes to Christmas cracker puns, she says, the feel-good factor is more probable to be triggered not by the joke in itself, but from the response to it.

"It's the laughter. The gag is the terrible Christmas cracker pun, and it's just a reason to chuckle together."

The Quest for the Ideal Festive Pun

Will we ever find the perfect gag?

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

Years ago, a professor established a research project for the world's most humorous joke.

More than 40,000 jokes later, with ratings lodged by 350,000 people globally, he has a clearer understanding than most as to what succeeds and what does not.

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

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

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

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

"The fascinating part about the holiday cracker jokes is that none of us considers them humorous.

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