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

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

We're at a joke-testing meeting with a company that makes products for gatherings. Its catalogue features Christmas crackers.

The firm's founder smiles, nearly sheepishly at the gag. But the joke has made the cut and will feature in future crackers.

"The success is gauged by the gag by the volume of moans and the loudness of the groans around the table," she explains.

The secret to a great holiday cracker pun is not the same as a stand-up gag per se. It is entirely about the setting - in this case, the shared amusement of the holiday dinner table with elders, children and potentially neighbours.

"You want the gag to be a thing that unites the child in harmony with the grandparent," she adds.

The Science Behind Shared Laughter

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

"Therefore when you are laughing with people around the Christmas table you are dropping into what's almost certainly a really ancient mammal social sound," explains a neuroscience expert.

Communal amusement, she says, helps make and maintain social bonds between people.

Scientists have found that a absence of such social exchanges can significantly harm mental and physical well-being.

"The people you converse with, and laugh with, it leads to increased levels of 'happy chemical' release," she adds.

Endorphins are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to enjoyable activities, such as chuckling with friends over a particularly terrible festive cracker gag.

"You're not just laughing at a silly pun with a holiday cracker," she states. "You are actually performing a lot of the really vital task of making, maintaining the connections you have with the people you love."

Which Happens In the Mind?

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

An awful lot occurs in reaction to humour, it transpires.

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

The research entails imaging the brains of volunteer subjects and then exposing them to a collection of humorous words, accompanied by either a non-emotional sound, or recorded laughter.

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

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

Combine all of this as a whole, and individuals listening to a joke have a sophisticated series of neural responses that support the laughter we hear.

The Contagious Power of Chuckles

Researchers discovered that when a funny phrase is paired with laughter there is a stronger reaction 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 move your face into a grin or a chuckle," the professor explains.

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

Laughter, says the professor, can be contagious.

So what does this imply for the chuckles heard at a Christmas table?

"You laugh more when you are familiar with people," she says, "and you laugh more when you like them or love them."

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

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

The Search for the Ideal Cracker Joke

Will we ever discover the perfect gag?

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

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

Over tens of thousands of jokes later, with scores lodged by hundreds of thousands of participants around the world, he has a better understanding than many as to what works and what does not.

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

"But they also be bad jokes, puns that cause us to moan," he continues.

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

"This is because if nobody finds it funny – it's the gag's shortcoming, not yours.

"What's interesting about the holiday cracker puns is that not one person considers them funny.

"It creates a shared moment at the table and I think it's lovely."