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

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

We're at a joke-testing meeting with a company that makes products for social events. Its catalogue includes Christmas crackers.

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

"You measure the gag by the number of moans and the loudness of the groans at the table," she says.

The secret to a great holiday cracker pun is not the identical as a good gag per se. It is all about the setting - in this case, the communal laughter of the holiday meal with grandparents, children and possibly friends.

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

The Science Behind Shared Laughter

Gathering to experience communal laughter is not only ancient, experts argue, it is probably to be older than humanity.

"Therefore when you are chuckling with people around the Christmas table you are engaging in what's very likely a truly primordial mammalian play vocalisation," says a professor.

Shared amusement, she explains, helps make and maintain social connections between people.

Scientists have discovered that a lack of these interactions can significantly harm mental and physical well-being.

"The people you converse with, and laugh with, it results in enhanced levels of 'happy chemical' release," she adds.

Endorphins are the body's "happy chemicals" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as chuckling with friends over a particularly awful festive cracker joke.

"You're not just chuckling at a silly joke with a holiday cracker," she says. "You are actually performing a lot of the really vital task of building, preserving the social bonds you have with those you love."

Which Happens In the Brain?

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

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

Using brain scanning technology, a kind of brain scanner which shows which areas of the mind are more active, scientists have been able to chart the areas that receive more blood.

Testing entails imaging the brains of volunteer subjects and then exposing them to a database of funny phrases, accompanied by either a non-emotional sound, or recorded chuckles.

"During the study we got a very fascinating activation pattern of activation," notes the neuroscientist.

A joke activates not just the parts of the mind responsible for auditory processing and interpreting language, but also brain regions associated with both preparation and initiating motion and those involved in sight and recall.

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

The Infectious Nature of Laughter

Scientists discovered that when a humorous word is paired with laughter there is a greater response in the mind than the same word when accompanied by a non-emotional sound.

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

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

Amusement, according to the expert, can be contagious.

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

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

When it comes to festive cracker puns, she says, the positive factor is more likely to be caused not by the gag itself, but from the reaction to it.

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

The Search for the Perfect Festive Pun

Is it possible to discover the ultimate joke?

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

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

More than 40,000 jokes submitted, with ratings provided by 350,000 participants around the world, he has a clearer understanding than most as to what succeeds and what does not.

The perfect festive cracker pun needs to be brief, he explains.

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

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

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

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

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