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

This joke is greeted with groans that resonate through a warehouse in the capital.

This describes a humor-evaluation session with a company that makes supplies for social events. Its repertoire features festive crackers.

The firm's founder smiles, almost apologetically at the gag. But the pun has made the cut and will appear in future crackers.

"You measure the joke by the volume of groans and the intensity of the groans around the table," the founder explains.

The secret to a good Christmas cracker pun is not the same as a good joke in itself. It is entirely about the setting - in this case, the shared amusement of the Christmas meal with grandparents, children and potentially friends.

"You want the gag to be something that unites the eight-year-old together with the 80-year-old," she adds.

The Neuroscience Of Communal Laughter

Gathering to experience shared amusement is not only nothing new, experts say, it is probably to be older than humanity.

"Therefore when you are chuckling with people around the holiday table you are engaging in what's almost certainly a really primordial mammal social sound," says a neuroscience expert.

Communal amusement, she says, helps forge and strengthen social bonds between people.

Researchers have found that a absence of these interactions can seriously harm mental and physical health.

"The people you talk to, and share laughter with, it results in enhanced levels of endorphin release," she adds.

These natural chemicals are the body's "feel-good compounds" and are released both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as laughing with loved ones over a particularly awful festive cracker gag.

"You're not just chuckling at a foolish joke with a Christmas cracker," she says. "You are in fact doing a lot of the truly important work of making, maintaining the connections you have with those you care about."

Which Happens In the Brain?

But what is truly taking place inside the brain when we hear a gag?

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

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

The research entails scanning the minds of volunteer subjects and then exposing them to a collection of funny words, paired with either a non-emotional sound, or recorded chuckles.

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

A joke stimulates not just the areas of the brain responsible for auditory processing and understanding speech, but also brain regions involved in both preparation and starting movement and those involved in vision and memory.

Combine all of this as a whole, and individuals hearing a joke have a sophisticated set of neural reactions that support the laughter we experience.

The Infectious Nature of Chuckles

Scientists found that when a humorous word is combined with chuckles there is a greater reaction in the brain than the identical word when accompanied by a neutral sound.

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

It indicates people are not just responding to funny words, they are responding to the amusement that accompanies them.

Laughter, says the expert, can be contagious.

So what does this mean for the chuckles heard around a Christmas table?

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

When it comes to Christmas cracker jokes, she says, the feel-good effect is more likely to be caused not by the joke in itself, but from the reaction to it.

"The laughter is key. The gag is the dreadful Christmas cracker joke, and it's just a pretext to laugh together."

The Search for the Ideal Festive Pun

Is it possible to discover the perfect gag?

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

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

More than tens of thousands of gags later, with scores lodged by 350,000 participants globally, he has a clearer idea than many as to what works and what does not.

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

"But they also need to be poor jokes, jokes that make us groan," he continues.

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

"This is because if no-one laughs – it's the joke's fault, not yours.

"The fascinating part about the Christmas cracker jokes is that not one person considers them funny.

"That's a common experience at the gathering and I think it's lovely."