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

This quip is greeted with groans that echo through a storage facility in the capital.

We're at a humor-evaluation session with a firm that produces products for social events. Its catalogue features festive crackers.

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

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

The secret to a good Christmas cracker pun is not the same as a good gag per se. It is all about the context - in this case, the shared amusement of the Christmas dinner table with grandparents, children and possibly friends.

"You want the joke to be something that unites the child in harmony with the grandparent," she states.

The Neuroscience Of Shared Amusement

Coming together to experience communal amusement is not only ancient, experts 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 almost certainly a really ancient mammalian play sound," explains a professor.

Communal amusement, she explains, aids in forge and strengthen social bonds between individuals.

Scientists have discovered that a lack of these social exchanges can significantly harm mental and physical health.

"The people you talk to, and laugh with, it leads to enhanced amounts of endorphin uptake," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in response to pleasurable activities, such as chuckling with friends over a truly terrible festive cracker gag.

"You're not just chuckling at a foolish pun with a holiday cracker," the expert says. "You are in fact performing a lot of the really important work of building, preserving the social bonds you have with those you love."

Which Occurs In the Brain?

But what is truly happening within the brain when we hear a joke?

An awful lot happens in reaction to humour, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the mind are more active, scientists have been able to map the regions that get more blood.

The research involves scanning the minds of volunteer participants and then exposing them to a database of funny phrases, paired with either a neutral sound, or pre-recorded laughter.

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

A gag activates not just the parts of the mind in charge of auditory processing and understanding language, but also brain areas associated with both preparation and initiating movement and those linked to vision and recall.

Put these elements together, and people listening to a joke have a sophisticated set of neural responses that underpin the laughter we hear.

The Contagious Power of Chuckles

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

"This activation occurred in parts of the brain that you would employ to contort your expression into a smile or a chuckle," she explains.

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

Laughter, says the professor, can be contagious.

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

"You laugh more when you are familiar with people," she notes, "and you laugh more when you are fond of them or care for them."

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

Is it possible to discover the ultimate joke?

Likely not, but that has not stopped experts from trying to.

In 2001, a professor established a research search for the world's most humorous gag.

Over tens of thousands of gags later, with ratings lodged by hundreds of thousands of participants around the world, he has a clearer understanding than most as to what succeeds and what fails.

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

"But they also need to be bad jokes, jokes that make us groan," he adds.

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

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

"The fascinating part about the holiday cracker puns is that not one person find them humorous.

"It creates a common experience around the table and I think it's wonderful."