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

This one-liner is met by moans that echo through a warehouse in the capital.

We're at a humor-evaluation meeting with a firm that makes supplies for gatherings. Its catalogue includes Christmas crackers.

The firm's founder grins, almost sheepishly at the gag. But the pun has been selected and will appear in future crackers.

"You measure the gag by the number of groans and the intensity of the groans around the table," she explains.

The secret to a good Christmas cracker pun is not the same as a stand-up joke per se. It is all about the context - in this case, the shared amusement of the Christmas dinner table with grandparents, kids and potentially neighbours.

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

The Neuroscience Of Communal Laughter

Gathering to experience communal laughter is not only nothing new, scientists say, it is probably to be older than humanity.

"Therefore when you are chuckling with others around the Christmas dinner you are dropping into what's almost certainly a really primordial mammal social vocalisation," says a professor.

Communal laughter, she explains, helps make and maintain social bonds between people.

Researchers have found that a lack of these social exchanges can seriously harm mental and physical health.

"The people you talk to, and share laughter with, it leads to enhanced amounts of endorphin release," she adds.

Endorphins are the brain's "feel-good compounds" and are released both to reduce tension and discomfort and in response to pleasurable experiences, such as chuckling with loved ones over a truly terrible Christmas cracker joke.

"It's not simply chuckling at a silly joke with a Christmas cracker," the expert says. "You are actually performing a lot of the really important work of making, maintaining the connections you have with the people you care about."

What Occurs In the Mind?

But what is truly taking place inside the mind when we hear a joke?

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

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 map the regions that receive more blood flow.

The research involves scanning the minds of healthy subjects and then subjecting them to a database of humorous words, paired with either a non-emotional sound, or pre-recorded laughter.

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

A gag stimulates not just the parts of the brain in charge of auditory processing and understanding language, but also neural areas involved in both planning and starting motion and those linked to vision and recall.

Put these elements as a whole, and people listening to a joke have a sophisticated set of neural responses that support the laughter we experience.

The Infectious Nature of Laughter

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

"This was in parts of the brain that you would use to move your face into a grin 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.

Amusement, says the expert, can be infectious.

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

"You laugh more when you know people," she notes, "and you laugh more when you like them or love them."

When it comes to festive cracker jokes, she says, the positive effect is more likely to be triggered not by the gag in itself, but from the reaction to it.

"It's the laughter. The gag is the dreadful Christmas cracker pun, and it's just a reason to chuckle together."

The Quest for the Ideal Festive Pun

Is it possible to find the ultimate gag?

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

In 2001, a psychologist established a research search for the planet's funniest joke.

Over 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 pun must be short, he says.

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

The increasingly "terrible" the gag, he states the more effective.

"The reason is that if no-one finds it funny – it's the gag's fault, not yours.

"What's interesting about the Christmas cracker puns is that not one person considers them humorous.

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