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

This joke is greeted with groans that resonate through a warehouse in the capital.

This describes a joke-testing meeting with a firm that makes products for gatherings. Its repertoire features Christmas crackers.

The firm's owner smiles, nearly sheepishly at the joke. But the joke has been selected and will appear in future crackers.

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

The secret to a great Christmas cracker pun is not the identical as a stand-up joke in itself. It is all about the context - in this instance, the communal amusement of the holiday meal with elders, kids and potentially neighbours.

"The goal is for the joke to be a thing that unites the child together with the grandparent," she adds.

The Neuroscience Of Communal Amusement

Gathering to enjoy communal amusement is not only ancient, scientists argue, it is likely to be pre-human.

"So when you are laughing with others around the holiday table you are engaging in what's almost certainly a really ancient mammalian social sound," explains a neuroscience expert.

Communal laughter, she says, helps make and maintain social connections between individuals.

Researchers have discovered that a lack of such interactions can significantly damage mental and physical health.

"Those you talk to, and share laughter with, it leads to enhanced levels of 'happy chemical' release," she adds.

These natural chemicals are the body's "feel-good compounds" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as laughing with friends over a particularly awful Christmas cracker gag.

"You're not just chuckling at a foolish joke with a Christmas cracker," she states. "You are in fact performing a lot of the really important task of making, maintaining the connections you have with those you care about."

What Occurs In the Mind?

But what is actually taking place inside the brain when we hear a joke?

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

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which parts of the mind are more active, scientists have been able to chart the areas that get more blood flow.

Testing entails scanning the brains of volunteer participants and then exposing them to a collection of humorous words, paired with either a non-emotional sound, or recorded chuckles.

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

A joke stimulates not just the areas of the brain responsible for hearing and understanding speech, but also neural regions involved in both preparation and starting movement and those involved in sight and recall.

Combine all of this together, and people hearing a joke have a complex series of neural responses that support the amusement we hear.

The Contagious Power of Laughter

Scientists discovered that when a humorous word is combined with chuckles there is a greater reaction in the brain than the identical phrase when accompanied by a neutral sound.

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

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

Amusement, according to the expert, can be infectious.

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

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

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

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

The Quest for the Perfect Festive Pun

Is it possible to find the perfect joke?

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

Years ago, a psychologist established a research project for the world's funniest joke.

More than tens of thousands of jokes later, with scores lodged by hundreds of thousands of people around the world, he has a better understanding than most as to what works and what fails.

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

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

The more "awful" the gag, he says the more effective.

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

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

"That's a shared moment at the table and I believe it's wonderful."