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"How much did Father Christmas's sled cost? Nothing, it was on the house."

This quip is met by groans that resonate through a warehouse in London.

This describes a humor-evaluation session with a firm that makes products for gatherings. Its catalogue includes festive crackers.

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

"The success is gauged by the joke by the volume of moans and the intensity of the groans around the table," she says.

The secret to a good Christmas cracker pun is not the same as a good joke in itself. It is entirely about the setting - in this instance, the shared amusement of the Christmas meal with elders, kids and possibly neighbours.

"The goal is for the joke to be something that brings the child in harmony with the grandparent," she states.

The Neuroscience Of Shared Laughter

Gathering to enjoy communal amusement is not only nothing new, experts argue, it is probably to be pre-human.

"So when you are chuckling with people at the Christmas table you are engaging in what's almost certainly a truly ancient mammal play vocalisation," says a neuroscience expert.

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

Researchers have discovered that a lack of such social exchanges can significantly harm mental and physical health.

"The people you talk to, and share laughter with, it leads to enhanced amounts of 'happy chemical' release," the professor adds.

Endorphins are the body's "happy chemicals" and are released both to alleviate stress and pain and in response to pleasurable activities, such as chuckling with friends over a particularly awful Christmas cracker gag.

"You're not just chuckling at a silly joke with a Christmas cracker," the expert states. "You are in fact performing a lot of the truly important work of building, preserving the connections you have with the people you care about."

What Happens In the Brain?

But what is truly happening within the mind when we hear a gag?

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

Employing brain scanning technology, a type of neural imager which shows which parts of the brain are more active, researchers have been able to chart the areas that receive more blood flow.

Testing involves imaging the minds of volunteer participants and then subjecting them to a collection of humorous phrases, accompanied by either a non-emotional sound, or pre-recorded laughter.

"During the study we observed a very interesting pattern of neural activity," says the professor.

A gag activates not just the parts of the brain responsible for auditory processing and understanding speech, but also brain regions involved in both planning and starting movement and those involved in vision and memory.

Put these elements together, and people hearing a pun have a complex set of neural reactions that underpin the amusement we experience.

The Contagious Power of Laughter

Researchers discovered that when a funny phrase is paired with laughter there is a greater response in the mind than the identical word when accompanied by a neutral sound.

"This activation occurred in areas of the mind that you would employ to contort your face into a grin or a laugh," she says.

It means we are not just responding to humorous words, they are responding to the laughter that follows them.

Amusement, according to the professor, can be infectious.

So what does this mean for the chuckles found around a holiday table?

"People laugh more when you know people," she says, "and you laugh more when you like them or love them."

When it comes to festive cracker puns, she says, the feel-good effect is more likely to be caused not by the joke in itself, but from the reaction to it.

"It's the laughter. The joke is the terrible Christmas cracker pun, and it's just a pretext to chuckle as a group."

The Search for the Ideal Cracker Joke

Is it possible to find the perfect joke?

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

Years ago, a professor set up a research project for the planet's funniest joke.

More than 40,000 gags submitted, with scores provided by hundreds of thousands of people globally, he has a clearer idea than many as to what works and what does not.

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

"But they also need to be bad gags, jokes that cause us to groan," he continues.

The increasingly "awful" the joke, he says the better.

"The reason is that if no-one laughs – it's the gag's shortcoming, not yours.

"The fascinating part about the Christmas cracker puns is that not one person find them humorous.

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