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"What was the price did Father Christmas's sleigh 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 joke-testing meeting with a firm that produces supplies for social events. Its catalogue features Christmas crackers.

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

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

The secret to a great Christmas cracker pun is not the same as a good gag in itself. It is all about the context - in this instance, the shared amusement of the Christmas meal with elders, children and possibly neighbours.

"You want the gag to be a thing that brings the eight-year-old in harmony with the grandparent," she states.

The Science Behind Communal Laughter

Coming together to experience shared amusement is not only ancient, scientists argue, it is probably to be older than humanity.

"Therefore when you are chuckling with others at the holiday table you are dropping into what's almost certainly a truly ancient mammal play vocalisation," explains a neuroscience expert.

Shared amusement, she says, helps forge and strengthen social connections between people.

Scientists have discovered that a lack of these interactions can seriously damage both psychological and bodily well-being.

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

Endorphins are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to pleasurable activities, such as laughing with loved ones over a particularly awful festive cracker gag.

"You're not just chuckling at a foolish joke with a holiday cracker," the expert states. "You are in fact performing a lot of the really important work of making, maintaining the connections you have with those you love."

Which Happens In the Brain?

But what is truly taking place within the brain when we listen to a joke?

A tremendous amount happens in reaction to comedy, it turns out.

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which parts of the brain are more active, scientists have been able to map the regions that receive more blood flow.

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

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

A gag activates not just the parts of the mind in charge of hearing and understanding language, but also neural areas associated with both planning and starting motion and those linked to sight and memory.

Combine all of this together, and people listening to a pun have a complex series of neural responses that support the amusement we hear.

The Contagious Nature of Chuckles

Researchers discovered that when a funny phrase is paired with laughter there is a greater response in the brain than the identical phrase when accompanied by a non-emotional sound.

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

It means people are not just responding to humorous words, they are responding to the amusement that follows them.

Amusement, according to the expert, can be infectious.

So what does this imply for the laughter heard around a holiday table?

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

When it comes to Christmas cracker puns, she explains, the feel-good effect is more probable to be triggered not by the gag itself, but from the reaction to it.

"It's the laughter. The joke is the terrible Christmas cracker joke, and it's just a pretext to chuckle together."

The Quest for the Ideal Cracker Joke

Will we ever find the perfect gag?

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

Years ago, a psychologist set up a scientific project for the world's most humorous joke.

Over tens of thousands of jokes later, with ratings lodged by 350,000 participants around the world, he has a better idea than most as to what succeeds and what does not.

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

"They must also need to be bad gags, puns that cause us to groan," he continues.

The increasingly "awful" the gag, he says the better.

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

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

"That's a shared experience at the table and I believe it's lovely."