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

This quip is met by groans that resonate through a warehouse in London.

We're at a humor-evaluation session with a firm that produces products for social events. Its repertoire includes Christmas crackers.

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

"The success is gauged by the joke by the volume of moans and the loudness of the groans at the table," she explains.

The secret to a great holiday cracker pun is not the same as a good joke in itself. It is entirely about the setting - in this instance, the communal laughter of the Christmas dinner table with elders, kids and potentially neighbours.

"The goal is for the joke to be something that unites the eight-year-old together with the 80-year-old," she adds.

The Neuroscience Behind Shared Amusement

Gathering to enjoy communal amusement is not only nothing new, scientists argue, it is probably to be older than humanity.

"So when you are laughing with people at the Christmas dinner you are engaging in what's very likely a really ancient mammal play sound," says a neuroscience expert.

Communal laughter, she says, helps forge and strengthen social bonds between individuals.

Scientists have discovered that a absence of such interactions can seriously damage mental and physical well-being.

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

These natural chemicals are the brain's "happy chemicals" and are released both to alleviate stress and pain and in response to enjoyable activities, such as laughing with loved ones over a particularly awful festive cracker joke.

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

Which Occurs In the Brain?

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

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

Employing brain scanning technology, a type of brain scanner which shows which parts of the brain are working harder, researchers have been able to chart the areas that get more blood.

The research involves imaging the minds of healthy participants and then subjecting them to a database of humorous words, paired with either a non-emotional sound, or recorded laughter.

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

A gag activates not just the areas of the brain responsible for auditory processing and interpreting speech, but also neural regions involved in both preparation and initiating movement and those linked to vision and recall.

Combine these elements together, and people listening to a pun have a sophisticated series of brain reactions that underpin the laughter we hear.

The Infectious Power of Laughter

Researchers discovered that when a humorous word is paired with chuckles there is a stronger reaction in the mind than the identical word when followed by a non-emotional sound.

"This was in parts of the mind that you would use to contort your expression into a smile or a chuckle," she says.

It means people 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 laughter heard at a Christmas gathering?

"People laugh more when you are familiar with people," she says, "and you laugh further when you are fond of them or love them."

When it comes to festive cracker jokes, she says, the positive factor is more likely to be caused not by the gag itself, but from the reaction to it.

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

The Search for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

In 2001, a psychologist set up a research project for the world's most humorous joke.

Over tens of thousands of jokes later, with scores provided by 350,000 people around the world, he has a better idea than most as to what works and what does not.

The perfect festive cracker joke must be short, he explains.

"They must also be poor jokes, jokes that cause us to moan," he continues.

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

"The reason is that if no-one finds it funny – it's the gag's shortcoming, not your own.

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

"That's a common moment at the gathering and I think it's wonderful."