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"What was the price did Father Christmas's sled cost? Nothing, 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 session with a company that makes supplies for gatherings. Its repertoire includes Christmas crackers.

The company's owner grins, almost sheepishly at the joke. But the joke has made the cut and will feature in future crackers.

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

The secret to a great Christmas cracker pun is not the identical as a stand-up gag per se. It is entirely about the context - in this instance, the communal amusement of the holiday meal with elders, children and potentially neighbours.

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

The Science Of Shared Amusement

Gathering to enjoy communal laughter is not only ancient, experts say, it is likely to be older than humanity.

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

Shared laughter, she explains, aids in make and maintain social bonds between individuals.

Scientists have found that a lack of such interactions can significantly damage mental and physical well-being.

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

These natural chemicals are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in reaction to enjoyable activities, such as chuckling with friends over a particularly awful festive cracker joke.

"It's not simply laughing at a foolish pun with a holiday cracker," she says. "You are actually doing a lot of the really important work of making, maintaining the connections you have with the people you care about."

What Happens In the Brain?

But what is actually happening within the mind when we listen to a joke?

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which areas of the brain are working harder, scientists have been able to chart the regions that get more blood flow.

Testing involves imaging the brains of healthy participants and then subjecting them to a collection of funny phrases, accompanied by either a neutral sound, or pre-recorded chuckles.

"In the scanner we got a very interesting activation pattern of neural activity," notes the neuroscientist.

A gag activates not just the parts of the mind responsible for hearing and interpreting language, but also neural regions associated with both preparation and starting motion and those linked to sight and memory.

Put all of this as a whole, and individuals hearing a pun have a complex set of neural responses that support the laughter we hear.

The Infectious Nature of Chuckles

Researchers found that when a funny phrase is paired with chuckles there is a stronger response in the brain than the same phrase when followed by a neutral sound.

"This was in areas of the brain that you would use to move your expression into a smile or a chuckle," the professor explains.

It means we are not just responding to funny jokes, they are reacting to the laughter that accompanies them.

Laughter, says the expert, can be infectious.

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

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

When it comes to Christmas cracker puns, she explains, the positive effect is more likely to be caused not by the gag in itself, but from the response to it.

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

The Quest for the Ideal Festive Pun

Is it possible to discover the perfect joke?

Probably not, but that has not stopped experts from attempting to.

Years ago, a professor established a research search for the planet's funniest joke.

More than 40,000 gags submitted, with scores provided by 350,000 participants around the world, he has a clearer idea than most as to what works and what fails.

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

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

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

"The reason is that if nobody laughs – it's the joke's shortcoming, not your own.

"The fascinating part about the Christmas cracker puns is that none of us find them humorous.

"That's a shared moment at the gathering and I think it's wonderful."