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"What was the price did Father Christmas's sleigh cost? Zero, it was on the house."

This one-liner is greeted with moans that echo through a warehouse in London.

We're at a humor-evaluation session with a company that makes supplies for social events. Its repertoire includes festive crackers.

The firm's founder smiles, almost sheepishly at the gag. But the pun has made the cut and will feature in upcoming crackers.

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

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

"The goal is for the gag to be something that unites the eight-year-old together with the 80-year-old," she adds.

The Neuroscience Behind Communal Laughter

Coming together to enjoy shared laughter is not only ancient, scientists say, it is likely to be pre-human.

"Therefore when you are chuckling with people at the Christmas dinner you are dropping into what's very likely a really primordial mammalian social sound," says a neuroscience expert.

Shared laughter, she explains, aids in make and maintain social connections between people.

Researchers have found that a lack of such interactions can seriously harm both psychological and bodily health.

"Those you converse with, and share laughter with, it results in increased levels of 'happy chemical' uptake," she adds.

Endorphins are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in response to pleasurable activities, such as chuckling with loved ones over a particularly terrible festive cracker joke.

"You're not just chuckling at a foolish pun with a holiday cracker," she states. "You are in fact doing a lot of the really important task of building, preserving the social bonds you have with the people you care about."

What Happens In the Brain?

But what is actually happening within the brain when we hear a joke?

A tremendous amount occurs in reaction to humour, it transpires.

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which shows which areas of the mind are working harder, researchers have been able to map the regions that get more blood.

The research involves scanning the brains of volunteer participants and then exposing them to a database of funny words, accompanied by either a neutral sound, or recorded laughter.

"During the study we got a really fascinating pattern of neural activity," says the neuroscientist.

A gag stimulates not just the parts of the brain responsible for hearing and understanding language, but also neural regions associated with both planning and starting motion and those involved in vision and recall.

Put these elements as a whole, and people hearing a pun have a complex series of neural responses that underpin the laughter we experience.

The Infectious Power of Chuckles

Scientists discovered that when a funny phrase is combined with laughter there is a greater response in the mind than the same word when followed by a neutral sound.

"This was in parts of the mind that you would employ to move your face into a smile or a laugh," the professor says.

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

Laughter, says the professor, can be contagious.

So what does this mean for the chuckles heard around a holiday gathering?

"People laugh harder when you are familiar with others," she says, "and you laugh more when you are fond of them or care for them."

When it comes to festive cracker jokes, she explains, the feel-good effect is more probable to be triggered not by the gag itself, but from the response to it.

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

The Search for the Perfect Cracker Joke

Will we ever discover the perfect joke?

Probably not, but that has not prevented experts from trying to.

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

Over 40,000 gags submitted, with ratings provided by 350,000 people around the world, he has a better understanding than many as to what succeeds and what does not.

The perfect Christmas cracker pun must be short, he says.

"They must also be bad gags, puns that make us groan," he adds.

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

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

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

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