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"How much did Santa's sled cost? Zero, it was on the house."

This joke is greeted with moans that echo through a storage facility in the capital.

This describes a humor-evaluation meeting with a firm that makes products for gatherings. Its catalogue includes Christmas crackers.

The firm's founder grins, nearly sheepishly at the joke. But the joke has been selected and will feature in future crackers.

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

The key to a good Christmas cracker pun is not the identical as a stand-up gag per se. It is entirely about the setting - in this instance, the shared laughter of the Christmas meal with grandparents, kids and potentially neighbours.

"The goal is for the joke to be something that unites the child in harmony with the grandparent," she adds.

The Science Of Shared Amusement

Coming together to enjoy shared laughter is not only nothing new, scientists argue, it is probably to be older than humanity.

"Therefore when you are laughing with people around the Christmas dinner you are dropping into what's very likely a really primordial mammalian social sound," says a professor.

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

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

"Those you converse with, and share laughter with, it results in increased levels of 'happy chemical' release," the professor continues.

Endorphins are the brain's "feel-good compounds" and are produced both to alleviate stress and pain and in reaction to pleasurable experiences, such as chuckling with friends over a particularly awful Christmas cracker gag.

"It's not simply laughing at a silly pun with a holiday cracker," the expert says. "You are in fact performing a lot of the really important task of making, maintaining the social bonds you have with the people you love."

Which Happens In the Brain?

But what is actually happening within the brain when we listen to a gag?

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

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

Testing involves imaging the minds of volunteer subjects and then subjecting them to a collection of humorous words, accompanied by either a neutral sound, or pre-recorded laughter.

"In the scanner we observed a very interesting pattern of neural activity," says the professor.

A gag activates not just the parts of the brain responsible for hearing and interpreting language, but also brain regions associated with both preparation and starting movement and those involved in sight and recall.

Put all of this as a whole, and individuals listening to a pun have a complex set of brain responses that underpin the amusement we experience.

The Infectious Nature of Laughter

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

"This activation occurred in parts of the brain that you would employ to contort your face into a smile or a chuckle," the professor says.

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

Laughter, according to the expert, can be infectious.

So what does this imply for the chuckles found around a holiday table?

"People laugh harder when you know people," she says, "and laughter increases more when you like them or love them."

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

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

The Quest for the Perfect Festive Pun

Will we ever discover the perfect gag?

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

In 2001, a psychologist set up a research project for the world's funniest gag.

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

The ideal Christmas cracker joke must be short, he explains.

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

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

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

"The fascinating part about the Christmas cracker puns is that none of us find them humorous.

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