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

This quip is met by moans that echo through a storage facility in the capital.

This describes a joke-testing session with a firm that produces supplies for gatherings. Its catalogue includes Christmas crackers.

The firm's owner smiles, nearly apologetically at the gag. But the joke has made the cut and will feature in upcoming crackers.

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

The secret to a great holiday cracker pun is not the identical as a good gag in itself. It is entirely about the context - in this instance, the communal laughter of the holiday meal with grandparents, children and potentially neighbours.

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

The Neuroscience Of Shared Amusement

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

"Therefore when you are chuckling with people at the Christmas table you are dropping into what's almost certainly a really primordial mammal play vocalisation," explains a professor.

Shared amusement, she explains, helps forge and strengthen social connections between individuals.

Scientists have discovered that a lack of such interactions can significantly damage both psychological and bodily well-being.

"The people you converse with, and share laughter with, it leads to enhanced levels of endorphin uptake," she continues.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce stress and pain and in reaction to enjoyable experiences, such as chuckling with loved ones over a truly terrible festive cracker joke.

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

What Happens In the Mind?

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

A tremendous amount happens in response to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which areas of the mind are working harder, researchers have been able to chart the areas that receive more blood flow.

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

"In the scanner we observed a really fascinating pattern of neural activity," says the professor.

A joke stimulates not just the parts of the mind responsible for hearing and interpreting language, but also brain areas involved in both planning and initiating movement and those involved in sight and recall.

Combine these elements as a whole, and people listening to a joke have a sophisticated series of neural reactions that support the amusement we hear.

The Contagious Power of Chuckles

Scientists found that when a funny phrase is paired with laughter there is a greater response in the brain than the same word when accompanied by a neutral sound.

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

It means people are not just reacting to funny jokes, they are reacting to the amusement that accompanies them.

Laughter, according to the professor, can be contagious.

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

"You laugh more when you know people," she says, "and you laugh more when you are fond of them or love them."

When it comes to festive cracker puns, she explains, the positive effect is more likely to be triggered not by the joke in 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 laugh as a group."

The Search for the Perfect Cracker Joke

Is it possible to discover the perfect gag?

Likely not, but that has not stopped experts from trying to.

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

More than 40,000 jokes later, with ratings provided by 350,000 participants globally, he has a better idea than most as to what works and what does not.

The perfect Christmas cracker pun must be brief, he explains.

"They must also need to be bad jokes, jokes that make us groan," he adds.

The increasingly "awful" the joke, he says the more effective.

"This is because if no-one finds it funny – it's the joke's shortcoming, not yours.

"What's interesting about the holiday cracker jokes is that not one person find them humorous.

"That's a common moment at the gathering and I believe it's lovely."