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"What was the price did Father Christmas's sled cost? Zero, it was on the house."

This one-liner is met by groans that echo through a warehouse in London.

This describes a joke-testing meeting with a firm that makes products for social events. Its catalogue features Christmas crackers.

The firm's founder smiles, nearly apologetically at the gag. But the joke has made the cut and will feature in future crackers.

"You measure the gag by the number of moans and the loudness of the groans at the table," the founder explains.

The key to a great Christmas cracker joke is not the identical as a stand-up gag per se. It is entirely about the context - in this case, the shared laughter of the holiday meal with elders, children and potentially friends.

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

The Neuroscience Behind Communal Laughter

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

"Therefore when you are laughing with others at the holiday dinner you are engaging in what's almost certainly a really ancient mammalian social vocalisation," explains a professor.

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

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

"The people you talk to, and laugh with, it leads to increased levels of endorphin uptake," the professor adds.

These natural chemicals are the body's "feel-good compounds" and are released both to alleviate stress and pain and in reaction to enjoyable activities, such as chuckling with loved ones over a truly terrible festive cracker gag.

"It's not simply laughing at a silly pun with a holiday cracker," the expert says. "You are actually doing a lot of the truly vital work of making, maintaining the connections you have with those you love."

What Occurs Inside the Brain?

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

An awful lot occurs in reaction to humour, it transpires.

Using brain scanning technology, a type of neural imager which shows which areas of the brain are working harder, scientists have been able to chart the regions that receive more blood flow.

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

"In the scanner we observed a really fascinating pattern of neural activity," notes the professor.

A joke activates not just the areas of the brain in charge of auditory processing and understanding language, but also brain areas associated with both planning and initiating motion 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 amusement we hear.

The Contagious Nature of Laughter

Scientists found that when a humorous word is paired with chuckles there is a greater response in the brain than the identical word when followed by a neutral sound.

"This was in parts of the brain that you would use to move your face into a grin or a laugh," the professor explains.

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

Amusement, says the professor, can be infectious.

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

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

When it comes to festive cracker jokes, she explains, the positive factor is more probable to be caused not by the gag in itself, but from the reaction to it.

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

The Search for the Perfect Cracker Joke

Will we ever find the perfect joke?

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

Years ago, a psychologist established a research project for the planet's most humorous joke.

Over 40,000 jokes submitted, with scores provided by hundreds of thousands of participants around the world, he has a clearer idea than most as to what succeeds and what fails.

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

"But they also need to be bad jokes, puns that make us groan," he continues.

The more "terrible" the joke, he states the better.

"The reason is that if nobody finds it funny – it's the gag's shortcoming, not yours.

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

"It creates a common experience around the table and I think it's lovely."