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

This one-liner is greeted with groans that echo through a storage facility in London.

We're at a joke-testing meeting with a company that makes supplies for gatherings. Its catalogue features festive crackers.

The firm's founder grins, almost apologetically at the gag. But the joke has made the cut and will appear in upcoming crackers.

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

The secret to a good Christmas cracker pun is not the identical as a good joke in itself. It is entirely about the context - in this instance, the communal laughter of the Christmas meal with elders, children and possibly friends.

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

The Neuroscience Behind Shared Amusement

Coming together to experience communal laughter is not only nothing new, scientists argue, it is probably to be pre-human.

"So when you are chuckling with people at the Christmas dinner you are engaging in what's almost certainly a truly ancient mammal social sound," says a professor.

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

Scientists have found that a lack of these social exchanges can seriously damage mental and physical health.

"Those you converse with, and laugh with, it results in enhanced amounts of 'happy chemical' release," she continues.

Endorphins are the brain's "feel-good compounds" and are released both to reduce stress and pain and in reaction to enjoyable activities, such as laughing with friends over a truly awful festive cracker gag.

"It's not simply chuckling at a silly joke with a holiday cracker," the expert says. "You are in fact doing a lot of the really important task of building, preserving the connections you have with those you care about."

What Occurs Inside the Brain?

But what is truly happening inside the brain when we hear a joke?

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

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which parts of the mind are working harder, researchers have been able to map the regions that receive more blood flow.

The research involves imaging the brains of healthy participants and then exposing them to a collection of humorous phrases, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we got a really interesting activation pattern of activation," says the professor.

A joke stimulates not just the parts of the mind in charge of auditory processing and interpreting language, but also neural areas involved in both planning and initiating movement and those involved in sight and memory.

Combine all of this together, and people listening to a pun have a complex set of brain reactions that support the laughter we hear.

The Contagious Power of Chuckles

Researchers discovered that when a humorous phrase is combined with laughter there is a greater reaction in the mind than the identical phrase when followed by a neutral sound.

"This was in parts of the brain that you would use to move your expression into a grin or a chuckle," she explains.

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

Amusement, says the expert, can be contagious.

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

"You laugh more when you know people," she notes, "and you laugh more when you like them or love them."

When it comes to Christmas cracker jokes, she explains, the feel-good effect is more likely to be triggered not by the gag in itself, but from the response to it.

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

The Search for the Perfect Cracker Joke

Is it possible to discover the ultimate gag?

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

In 2001, a psychologist set up a research search for the planet's most humorous gag.

More than 40,000 jokes submitted, with ratings lodged by hundreds of thousands of participants around the world, he has a better understanding than most as to what works and what fails.

The ideal Christmas cracker joke needs to be brief, he says.

"But they also be poor jokes, puns that cause us to groan," he continues.

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

"This is because if nobody laughs – it's the joke's shortcoming, not yours.

"What's interesting about the Christmas cracker jokes is that not one person find them funny.

"It creates a common moment around the gathering and I think it's wonderful."