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

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

This describes a humor-evaluation meeting with a firm that makes supplies for social events. Its catalogue features Christmas crackers.

The firm's founder grins, almost apologetically at the gag. But the joke has been selected and will appear in future crackers.

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

The key to a good Christmas cracker joke is not the same as a good joke per se. It is entirely about the context - in this instance, the communal amusement of the holiday dinner table with grandparents, children and possibly friends.

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

The Neuroscience Of Communal Laughter

Coming together to experience communal amusement is not only nothing new, experts argue, it is likely to be pre-human.

"So when you are laughing with people around the holiday dinner you are engaging in what's almost certainly a really primordial mammalian play vocalisation," says a professor.

Shared laughter, she says, helps forge and strengthen social bonds between people.

Researchers have discovered that a absence of these social exchanges can seriously damage mental and physical well-being.

"The people you talk to, and share laughter with, it leads to increased amounts of 'happy chemical' uptake," she adds.

Endorphins are the body's "happy chemicals" and are released both to alleviate stress and pain and in response to pleasurable activities, such as laughing with loved ones over a truly awful Christmas cracker joke.

"You're not just chuckling at a silly pun with a holiday cracker," she says. "You are in fact doing a lot of the really important task of making, maintaining the social bonds you have with the people you love."

What Occurs In the Brain?

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

An awful lot happens in reaction to humour, it turns out.

Employing brain scanning technology, a type of neural imager which shows which areas of the mind are more active, scientists have been able to chart the regions that receive more blood flow.

The research entails scanning the minds of healthy participants and then exposing them to a collection of humorous phrases, paired with either a neutral sound, or pre-recorded chuckles.

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

A gag stimulates not just the areas of the brain responsible for auditory processing and understanding speech, but also neural areas associated with both preparation and initiating motion and those involved in vision and memory.

Combine all of this together, and individuals listening to a pun have a complex series of brain responses that underpin the laughter we hear.

The Contagious Power of Chuckles

Scientists found that when a funny word is paired with laughter there is a greater reaction in the mind than the identical phrase when followed by a non-emotional sound.

"This activation occurred in areas of the mind that you would employ to contort your expression into a smile or a chuckle," she says.

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

Laughter, says the professor, can be infectious.

So what does this mean for the laughter found at a holiday table?

"People laugh more when you are familiar with others," she says, "and you laugh more when you like them or care for them."

When it comes to festive cracker jokes, she says, the positive effect is more probable to be triggered not by the joke itself, but from the response to it.

"The laughter is key. The gag is the terrible holiday cracker pun, and it's just a pretext to chuckle as a group."

The Search for the Perfect Cracker Joke

Will we ever discover the ultimate gag?

Likely not, but that has not stopped researchers from attempting to.

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

Over tens of thousands of gags submitted, with scores lodged by hundreds of thousands of participants around the world, he has a better understanding than most as to what succeeds and what does not.

The perfect Christmas cracker pun must be brief, he says.

"But they also need to be poor jokes, puns that make us moan," he adds.

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

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

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

"That's a common moment at the gathering and I believe it's lovely."