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"How much did Santa's sled cost? Nothing, it was on the house."

This one-liner is greeted with moans that resonate through a storage facility in London.

This describes a joke-testing session with a firm that makes supplies for gatherings. Its catalogue includes Christmas crackers.

The firm's owner smiles, almost apologetically at the joke. But the pun has made the cut and will feature in future crackers.

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

The key to a great Christmas cracker joke is not the same as a good gag per se. It is entirely about the context - in this case, the communal laughter of the Christmas dinner table with elders, children and potentially neighbours.

"The goal is for the joke to be something that brings the eight-year-old together with the grandparent," she adds.

The Neuroscience Behind Communal Laughter

Gathering to experience communal laughter is not only ancient, scientists argue, it is probably to be older than humanity.

"So when you are chuckling with others around the holiday table you are dropping into what's very likely a truly primordial mammalian social sound," says a neuroscience expert.

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

Scientists have found that a lack of such interactions can seriously harm both psychological and bodily well-being.

"Those you converse with, and laugh with, it results in increased amounts of endorphin release," the professor adds.

Endorphins are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to enjoyable experiences, such as chuckling with friends over a particularly awful festive cracker gag.

"It's not simply chuckling at a foolish pun with a Christmas cracker," she says. "You are in fact performing a lot of the really important work of making, maintaining the social bonds you have with those you care about."

What Happens In the Mind?

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

A tremendous amount occurs in response to humour, it turns out.

Employing brain scanning technology, a type of brain scanner which shows which areas of the brain are working harder, researchers have been able to map the regions that receive more blood.

The research entails imaging the brains of volunteer subjects and then exposing them to a database of funny words, paired with either a non-emotional sound, or pre-recorded chuckles.

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

A joke activates not just the areas of the mind in charge of hearing and understanding language, but also brain regions associated with both preparation and starting motion and those involved in vision and recall.

Combine all of this as a whole, and individuals listening to a joke have a sophisticated set of neural responses that support the laughter we experience.

The Infectious Nature of Chuckles

Scientists found that when a humorous phrase is paired with laughter there is a stronger reaction in the brain than the identical word when accompanied by a neutral sound.

"This was in areas of the mind that you would employ to move your face into a smile or a laugh," the professor says.

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

Laughter, according to the expert, can be infectious.

So what does this mean for the laughter heard at a holiday gathering?

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

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

"The laughter is key. The gag is the dreadful Christmas cracker joke, and it's just a pretext to chuckle together."

The Quest for the Ideal Cracker Joke

Will we ever discover the ultimate gag?

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

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

More than tens of thousands of gags later, with ratings provided by hundreds of thousands of people globally, he has a better idea than many as to what works and what fails.

The ideal festive cracker joke must be brief, he says.

"But they also need to be poor gags, jokes that cause us to moan," he adds.

The more "awful" the joke, he states the more effective.

"This is because if nobody finds it funny – it's the joke's fault, not your own.

"The fascinating part about the holiday cracker jokes is that not one person considers them humorous.

"It creates a shared experience around the gathering and I believe it's wonderful."