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

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

We're at a humor-evaluation meeting with a company that makes products for social events. Its repertoire features festive crackers.

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

"The success is gauged by the joke by the number of moans and the intensity of the groans at the table," the founder says.

The secret to a good holiday cracker joke is not the same as a good gag in itself. It is entirely about the context - in this case, the shared laughter of the Christmas meal with elders, kids and possibly friends.

"You want the joke to be something that unites the child together with the grandparent," she adds.

The Neuroscience Of Shared Amusement

Gathering to experience shared laughter is not only nothing new, scientists argue, it is probably to be pre-human.

"So when you are chuckling with people around the holiday table you are engaging in what's very likely a truly primordial mammal play sound," says a neuroscience expert.

Communal laughter, she says, helps forge and strengthen social connections between people.

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

"The people you converse with, and laugh with, it results in enhanced levels of endorphin release," the professor adds.

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

"You're not just laughing at a foolish pun with a Christmas cracker," she states. "You are in fact doing a lot of the truly vital task of making, maintaining the connections you have with those you care about."

Which Happens In the Brain?

But what is truly taking place within the mind when we hear a joke?

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

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

Testing involves imaging the brains of volunteer participants and then subjecting them to a database of humorous words, paired with either a neutral sound, or pre-recorded laughter.

"During the study we got a really interesting pattern of activation," says the neuroscientist.

A joke stimulates not just the areas of the mind in charge of hearing and interpreting language, but also neural regions associated with both planning and starting motion and those involved in sight and memory.

Put all of this together, and people listening to a joke have a complex set of brain responses that underpin the laughter we experience.

The Infectious Nature of Laughter

Scientists found that when a humorous word is combined with laughter there is a stronger response in the brain than the same word when followed by a non-emotional sound.

"This activation occurred in parts of the mind that you would employ to move your expression into a grin or a chuckle," she explains.

It means we are not just responding to funny words, they are responding to the laughter that follows them.

Amusement, according to the professor, can be contagious.

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

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

When it comes to festive cracker puns, she explains, the feel-good factor is more probable to be caused not by the gag in itself, but from the reaction to it.

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

The Search for the Perfect Cracker Joke

Is it possible to discover the perfect joke?

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

Years ago, a professor set up a scientific search for the planet's most humorous gag.

Over tens of thousands of gags submitted, with scores provided by 350,000 people globally, he has a clearer understanding than most as to what succeeds and what does not.

The ideal Christmas cracker pun needs to be short, he explains.

"But they also be poor gags, jokes that cause us to moan," he adds.

The increasingly "terrible" the gag, he states the more effective.

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

"What's interesting about the holiday cracker puns is that not one person considers them funny.

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