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

This one-liner is greeted with groans that echo through a warehouse in the capital.

This describes a humor-evaluation meeting with a company that produces products for social events. Its repertoire features festive crackers.

The company's founder grins, almost apologetically at the joke. But the pun has been selected and will feature in upcoming crackers.

"You measure the gag by the volume of moans and the intensity of the groans around the table," she says.

The key to a great holiday cracker joke is not the identical as a good gag in itself. It is entirely about the setting - in this instance, the communal amusement of the holiday meal with elders, children and possibly friends.

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

The Science Behind Shared Amusement

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

"So when you are laughing with people around the Christmas dinner you are engaging in what's almost certainly a truly primordial mammal social vocalisation," says a professor.

Shared amusement, she explains, aids in make and maintain social bonds between people.

Scientists have found that a lack of these social exchanges can seriously harm both psychological and bodily health.

"Those you talk to, and share laughter with, it results in increased levels of 'happy chemical' uptake," she adds.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in response to enjoyable experiences, such as chuckling with loved ones over a truly terrible festive cracker joke.

"You're not just laughing at a silly pun with a Christmas cracker," the expert states. "You are in fact performing a lot of the truly vital task of building, preserving the social bonds you have with those you care about."

What Occurs Inside the Mind?

But what is actually taking place within the brain when we hear a gag?

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

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

Testing involves scanning the minds of healthy participants and then subjecting them to a collection of funny phrases, accompanied by either a neutral sound, or recorded laughter.

"In the scanner we observed a really interesting activation pattern of neural activity," notes the neuroscientist.

A joke stimulates not just the parts of the mind in charge of hearing and understanding language, but also brain areas involved in both planning and initiating movement and those involved in sight and recall.

Combine all of this as a whole, and people hearing a joke have a complex set of neural reactions that underpin the amusement we hear.

The Contagious Nature of Chuckles

Scientists found that when a humorous phrase is paired with laughter there is a stronger reaction in the mind than the same phrase when followed by a neutral sound.

"This was in areas of the brain that you would employ to move your expression into a smile or a laugh," the professor says.

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

Amusement, says the expert, can be infectious.

So what does this imply for the laughter heard around a Christmas table?

"You laugh more when you know others," she notes, "and you laugh further when you are fond of them or love them."

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

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

The Search for the Perfect Cracker Joke

Will we ever find the perfect joke?

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

Years ago, a psychologist set up a scientific search for the world's funniest gag.

Over 40,000 gags submitted, with ratings lodged by hundreds of thousands of participants globally, he has a better understanding than most as to what succeeds and what fails.

The perfect Christmas cracker pun needs to be short, he says.

"They must also need to be poor gags, jokes that cause us to groan," he adds.

The increasingly "awful" the gag, he states the better.

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

"The fascinating part about the Christmas cracker puns is that not one person find them humorous.

"That's a shared experience around the gathering and I think it's wonderful."