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"What was the price did Santa's sleigh cost? Nothing, it was on the house."

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

This describes a humor-evaluation meeting with a company that produces products for social events. Its repertoire includes Christmas crackers.

The firm's founder grins, nearly sheepishly at the gag. But the joke has been selected and will feature in future crackers.

"You measure the gag by the volume of moans and the intensity of the groans around the table," she says.

The key to a good Christmas cracker pun is not the identical as a good gag in itself. It is entirely about the setting - in this case, the shared laughter of the holiday meal with grandparents, kids and potentially neighbours.

"The goal is for the gag to be a thing that unites the child in harmony with the 80-year-old," she states.

The Neuroscience Behind Shared Amusement

Coming together to enjoy shared amusement is not only nothing new, scientists say, it is likely to be pre-human.

"So when you are laughing with people at the Christmas dinner you are engaging in what's almost certainly a truly ancient mammal social sound," explains a neuroscience expert.

Communal amusement, she explains, aids in forge and strengthen social bonds between individuals.

Researchers have found that a lack of these social exchanges can significantly damage both psychological and bodily health.

"Those you converse with, and share laughter with, it leads to increased levels of endorphin release," the professor continues.

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

"You're not just chuckling at a silly joke with a holiday cracker," the expert says. "You are actually performing a lot of the really vital work of building, preserving the connections you have with the people you care about."

Which Happens Inside the Mind?

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

An awful lot happens in response to comedy, it turns out.

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

Testing involves scanning the minds of volunteer subjects and then exposing them to a database of humorous phrases, accompanied by either a non-emotional sound, or recorded laughter.

"During the study we observed a really fascinating pattern of neural activity," says the professor.

A joke activates not just the parts of the mind in charge of auditory processing and interpreting speech, but also neural regions involved in both planning and initiating motion and those linked to vision and recall.

Put all of this as a whole, and people listening to a pun have a complex set of brain responses that support the laughter we hear.

The Infectious Nature of Chuckles

Scientists found that when a humorous phrase is paired with laughter there is a greater response in the mind than the identical phrase when followed by a neutral sound.

"This activation occurred in areas of the mind that you would employ to contort your expression into a smile or a laugh," she explains.

It indicates people are not just responding to humorous jokes, they are reacting to the laughter that accompanies them.

Amusement, says the professor, can be contagious.

So what does this imply for the chuckles found at a Christmas 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 Christmas cracker jokes, she says, the feel-good factor 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 terrible Christmas cracker pun, and it's just a pretext to laugh together."

The Quest for the Perfect Festive Pun

Is it possible to find the ultimate gag?

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

Years ago, a professor set up a research search for the world's funniest joke.

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

The perfect Christmas cracker joke must be short, he explains.

"They must also be poor gags, puns that cause us to moan," he adds.

The more "awful" the joke, he states the better.

"This is because if no-one laughs – it's the gag's fault, not your own.

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

"It creates a common experience around the gathering and I think it's wonderful."