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

This one-liner is greeted with groans that resonate through a warehouse in London.

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

The firm's founder grins, nearly sheepishly at the joke. But the joke has made the cut and will feature in upcoming crackers.

"You measure the joke by the volume of groans and the intensity of the groans at the table," she explains.

The key to a good Christmas cracker joke is not the same as a good joke in itself. It is all about the context - in this case, the shared amusement of the holiday dinner table with elders, children and potentially friends.

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

The Neuroscience Of Communal Laughter

Coming together to enjoy shared laughter is not only nothing new, scientists say, it is likely to be pre-human.

"So when you are laughing with others at the Christmas dinner you are dropping into what's almost certainly a truly primordial mammalian play sound," says a professor.

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

Scientists have found that a lack of these interactions can seriously damage both psychological and bodily health.

"The people you converse with, and share laughter with, it leads to increased amounts of 'happy chemical' release," the professor 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 friends over a particularly terrible festive cracker joke.

"You're not just laughing at a silly pun with a holiday cracker," she says. "You are in fact doing a lot of the truly vital work of building, preserving the connections you have with the people you care about."

Which Occurs Inside the Mind?

But what is actually taking place within the brain when we listen to a joke?

An awful lot occurs in reaction to humour, it turns out.

Employing brain scanning technology, a kind of neural imager which shows which areas of the mind are more active, researchers have been able to map the regions that get more blood.

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

"During the study we got a very interesting pattern of activation," notes the neuroscientist.

A joke activates not just the areas of the mind in charge of hearing and understanding language, but also neural regions involved in both planning and initiating motion and those involved in sight and recall.

Put all of this as a whole, and individuals listening to a joke have a sophisticated set of brain reactions that support the laughter we hear.

The Infectious Power of Laughter

Scientists discovered that when a humorous word is paired with chuckles there is a greater response in the brain than the identical phrase when followed by a non-emotional sound.

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

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

Laughter, says the expert, can be contagious.

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

"You laugh more when you are familiar with people," she notes, "and you laugh further when you like them or care for them."

When it comes to Christmas cracker puns, she says, the feel-good effect is more probable to be triggered not by the gag itself, but from the reaction to it.

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

The Search for the Perfect Cracker Joke

Is it possible to find the ultimate joke?

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

In 2001, a professor set up a scientific search for the planet's funniest gag.

Over 40,000 jokes submitted, with scores lodged by hundreds of thousands of people around the world, he has a better understanding than many as to what works and what fails.

The ideal Christmas cracker pun needs to be brief, he explains.

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

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

"The reason is that if no-one finds it funny – it's the joke's shortcoming, not your own.

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

"That's a common moment around the gathering and I believe it's lovely."