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

This one-liner is greeted with moans that resonate through a warehouse in London.

We're at a humor-evaluation meeting with a firm that makes supplies for social events. Its catalogue features Christmas crackers.

The company's founder smiles, almost sheepishly at the gag. But the joke has been selected and will appear in upcoming crackers.

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

The key to a great Christmas cracker joke is not the identical as a stand-up gag per se. It is all about the context - in this case, the communal laughter of the Christmas meal with elders, children and potentially friends.

"You want the joke to be something that brings the eight-year-old in harmony with the 80-year-old," she adds.

The Neuroscience Behind Communal 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 chuckling with others at the Christmas dinner you are dropping into what's very likely a really ancient mammalian play sound," says a professor.

Shared amusement, she says, aids in make and maintain social connections between individuals.

Scientists have found that a absence of these interactions can seriously harm both psychological and bodily well-being.

"Those you talk to, and laugh with, it results in enhanced amounts of 'happy chemical' uptake," she continues.

Endorphins are the brain's "feel-good compounds" and are produced both to alleviate stress and pain and in response to pleasurable experiences, such as laughing with loved ones over a truly terrible festive cracker gag.

"It's not simply chuckling at a foolish joke with a Christmas cracker," she says. "You are actually performing a lot of the really important work of building, preserving the social bonds you have with the people you love."

What Happens Inside the Mind?

But what is truly happening inside the mind when we hear a gag?

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which areas of the mind are more active, researchers have been able to chart the regions that get more blood.

Testing entails scanning the brains of healthy subjects and then exposing them to a database of funny words, paired with either a non-emotional sound, or recorded laughter.

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

A joke stimulates not just the parts of the mind in charge of auditory processing and understanding language, but also brain regions associated with both preparation and initiating movement and those involved in vision and recall.

Put these elements as a whole, and individuals hearing a joke have a complex series of neural responses that underpin the laughter we experience.

The Contagious Power of Laughter

Scientists discovered that when a humorous word is paired with laughter there is a greater response in the brain than the identical phrase when accompanied by a neutral sound.

"This was in parts of the mind that you would use to move your face into a smile or a laugh," she explains.

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

Amusement, says the professor, can be infectious.

So what does this mean for the chuckles heard at a holiday table?

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

When it comes to festive cracker puns, she explains, the positive effect is more probable to be triggered not by the joke itself, but from the reaction to it.

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

The Quest for the Ideal Cracker Joke

Will we ever discover the perfect joke?

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

In 2001, a psychologist established a research search for the planet's most humorous joke.

Over 40,000 gags submitted, with ratings lodged by 350,000 people around the world, he has a clearer understanding than most as to what works and what fails.

The perfect Christmas cracker pun must be short, he explains.

"They must also be bad jokes, jokes that make us groan," he adds.

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

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

"What's interesting about the Christmas cracker puns is that none of us find them humorous.

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