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

This one-liner is greeted with groans that echo through a storage facility in the capital.

We're at a joke-testing session with a company that makes supplies for gatherings. Its catalogue features festive crackers.

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

"You measure the gag by the number of groans and the loudness of the groans at the table," she explains.

The key to a good Christmas cracker pun is not the same as a stand-up joke per se. It is all about the setting - in this instance, the communal laughter of the Christmas meal with elders, children and possibly friends.

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

The Science Of Shared Laughter

Coming together to enjoy shared laughter is not only ancient, experts argue, it is probably to be pre-human.

"So when you are chuckling with people around the holiday table you are engaging in what's almost certainly a truly primordial mammalian social sound," says a neuroscience expert.

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

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

"Those you converse with, 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 alleviate stress and pain and in reaction to enjoyable activities, such as laughing with loved ones over a particularly awful festive cracker gag.

"You're not just chuckling at a foolish joke with a holiday cracker," she states. "You are actually performing a lot of the really vital work of building, preserving the connections you have with those you love."

What Happens In the Brain?

But what is truly happening inside the mind when we listen to a joke?

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

Employing functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which parts of the mind are more active, scientists have been able to chart the regions that get more blood flow.

The research entails imaging the brains of volunteer subjects and then subjecting them to a database of humorous phrases, paired with either a non-emotional sound, or pre-recorded laughter.

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

A gag stimulates not just the areas of the mind in charge of hearing and interpreting language, but also neural regions associated with both planning and starting movement and those linked to sight and recall.

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

The Infectious Nature of Laughter

Scientists found that when a humorous phrase is combined with laughter there is a stronger response in the brain than the same phrase when followed by a non-emotional sound.

"This was in areas of the mind that you would employ to contort your face into a smile or a laugh," the professor explains.

It indicates people are not just responding to humorous words, they are reacting to the amusement that follows them.

Amusement, says the expert, can be infectious.

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

"People laugh more when you know others," she notes, "and laughter increases further when you are fond of them or care for them."

When it comes to Christmas cracker puns, she explains, the feel-good factor is more likely to be triggered not by the gag 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 laugh as a group."

The Quest for the Perfect Festive Pun

Will we ever discover the perfect gag?

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

In 2001, a professor set up a research search for the world's most humorous gag.

More than tens of thousands of gags later, with ratings lodged by hundreds of thousands of participants around the world, he has a better idea than most as to what succeeds and what does not.

The ideal festive cracker pun needs to be short, he says.

"They must also be bad gags, jokes that make us moan," he adds.

The more "awful" the gag, he says the more effective.

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

"The fascinating part about the holiday cracker jokes is that none of us considers them funny.

"That's a shared experience at the table and I believe it's wonderful."