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

This one-liner is met by moans that echo through a warehouse in the capital.

We're at a joke-testing session with a company that makes products for gatherings. Its repertoire features Christmas crackers.

The firm's founder grins, nearly apologetically at the joke. But the pun has made the cut and will appear in future crackers.

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

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 communal amusement of the Christmas dinner table with elders, kids and possibly friends.

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

The Neuroscience Of Shared Amusement

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

"So when you are chuckling with people around the holiday dinner you are engaging in what's almost certainly a really ancient mammal play vocalisation," explains a professor.

Shared amusement, she says, helps make and maintain social bonds between individuals.

Scientists have found that a lack of these social exchanges can significantly harm both psychological and bodily well-being.

"The people you talk to, and share laughter with, it results in increased amounts of endorphin release," she adds.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce stress and pain and in response to enjoyable activities, such as chuckling with loved ones over a particularly terrible Christmas cracker gag.

"It's not simply chuckling at a silly pun with a holiday cracker," the expert states. "You are actually doing a lot of the really important work of making, maintaining the connections you have with those you care about."

What Happens Inside the Brain?

But what is truly happening inside the mind when we hear a gag?

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

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which parts of the mind are working harder, researchers have been able to map the areas that get more blood.

Testing involves imaging the brains of volunteer participants and then exposing them to a collection of humorous words, accompanied by either a neutral sound, or pre-recorded chuckles.

"In the scanner we observed a very fascinating pattern of activation," says the professor.

A joke stimulates not just the parts of the brain in charge of auditory processing and interpreting speech, but also neural areas associated with both planning and initiating motion and those involved in vision and recall.

Put these elements together, and individuals hearing a pun have a complex set of brain reactions that support the amusement we experience.

The Contagious Nature of Laughter

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

"This was in areas of the brain that you would employ to move your expression into a grin or a chuckle," the professor explains.

It means we are not just reacting to humorous jokes, they are reacting to the amusement that follows them.

Amusement, says the expert, can be contagious.

So what does this mean for the chuckles found at a holiday table?

"You laugh harder when you know people," she says, "and you laugh further when you like them or care for them."

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

"The laughter is key. The gag is the dreadful Christmas cracker pun, and it's just a reason to laugh as a group."

The Quest for the Ideal Cracker Joke

Is it possible to find the ultimate joke?

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

In 2001, a professor set up a scientific search for the world's funniest joke.

Over tens of thousands of jokes submitted, with ratings provided by 350,000 people 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.

"They must also need to be bad jokes, jokes that cause us to moan," he continues.

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

"The reason is that if nobody laughs – it's the gag's shortcoming, not your own.

"What's interesting about the holiday cracker puns is that none of us find them funny.

"That's a common moment at the gathering and I believe it's wonderful."