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

This one-liner is greeted with groans that echo through a storage facility in London.

This describes a joke-testing meeting with a company that produces products for social events. Its catalogue includes Christmas crackers.

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

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

The key to a good holiday cracker joke is not the identical as a good joke per se. It is all about the context - in this instance, the shared laughter of the holiday dinner table with grandparents, children and possibly neighbours.

"You want the joke to be something that unites the eight-year-old together with the 80-year-old," she states.

The Science Behind Communal Amusement

Gathering to enjoy shared amusement is not only ancient, experts say, it is probably to be pre-human.

"Therefore when you are chuckling with people at the Christmas dinner you are dropping into what's almost certainly a really primordial mammal social sound," says a professor.

Communal laughter, she explains, aids in forge and strengthen social connections between individuals.

Scientists have discovered that a absence of such interactions can significantly harm mental and physical health.

"The people you talk to, and share laughter with, it leads to enhanced levels of 'happy chemical' release," the professor continues.

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

"You're not just laughing at a foolish joke with a Christmas cracker," she says. "You are actually doing a lot of the truly important task of building, preserving the social bonds you have with those you love."

What Happens In the Mind?

But what is truly happening inside the brain when we listen to a joke?

A tremendous amount occurs in response to comedy, it turns out.

Using brain scanning technology, a kind of brain scanner which indicates which areas of the mind are more active, scientists have been able to map the areas that receive more blood.

Testing involves scanning the brains of healthy subjects and then exposing them to a collection of humorous words, accompanied by either a non-emotional sound, or pre-recorded laughter.

"During the study we observed a very interesting pattern of neural activity," notes the professor.

A joke stimulates not just the parts of the mind responsible for hearing and interpreting language, but also brain regions associated with both preparation and starting movement and those linked to vision and recall.

Put these elements together, and individuals listening to a joke have a complex series of neural responses that underpin the laughter we experience.

The Contagious Nature of Laughter

Scientists discovered that when a funny word is combined with laughter there is a greater reaction in the brain than the same phrase when followed by a neutral sound.

"This was in parts of the brain that you would employ to move your face into a smile or a laugh," the professor explains.

It indicates we are not just reacting to humorous jokes, they are reacting to the laughter that accompanies them.

Amusement, says the expert, can be contagious.

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

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

When it comes to Christmas cracker puns, she says, the positive effect is more probable to be caused not by the gag itself, but from the reaction to it.

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

The Quest for the Ideal Festive Pun

Will we ever find the perfect joke?

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

Years ago, a psychologist set up a scientific search for the planet's funniest joke.

More than tens of thousands of jokes submitted, with scores provided by hundreds of thousands of participants around the world, he has a clearer idea than many as to what succeeds and what does not.

The perfect festive cracker pun must be brief, he explains.

"But they also need to be bad gags, jokes that make us moan," he continues.

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

"This is because if nobody finds it funny – it's the joke's fault, not your own.

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

"It creates a common experience around the gathering and I think it's lovely."