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

This joke is greeted with groans that echo through a warehouse in London.

We're at a joke-testing session with a firm that produces products for gatherings. Its catalogue features festive crackers.

The firm's owner smiles, nearly apologetically at the joke. But the joke has been selected and will feature in future crackers.

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

The secret to a good holiday cracker pun is not the identical as a good gag per se. It is entirely about the context - in this instance, the shared laughter of the holiday meal with elders, kids and potentially friends.

"You want the joke to be a thing that unites the child in harmony with the 80-year-old," she states.

The Neuroscience Of Shared Laughter

Coming together to experience communal amusement is not only ancient, scientists argue, it is likely to be older than humanity.

"So when you are chuckling with people around the Christmas dinner you are dropping into what's very likely a truly primordial mammalian play vocalisation," explains a professor.

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

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

"The people you converse with, and laugh with, it leads to enhanced levels of 'happy chemical' 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 pleasurable activities, such as chuckling with loved ones over a particularly awful Christmas cracker joke.

"It's not simply chuckling at a foolish pun with a Christmas cracker," the expert states. "You are in fact doing a lot of the really important task of building, preserving the connections you have with the people you love."

Which Happens In the Mind?

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

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

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

Testing entails imaging the brains of volunteer subjects and then subjecting them to a collection of humorous phrases, paired with either a neutral sound, or pre-recorded laughter.

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

A joke activates not just the parts of the brain in charge of hearing and understanding language, but also neural areas associated with both planning and initiating movement and those involved in vision and recall.

Combine these elements together, and people listening to a joke have a complex set of brain responses that underpin the amusement we experience.

The Contagious Power of Laughter

Researchers discovered that when a humorous word is combined with chuckles there is a greater response in the mind than the identical word when followed by a non-emotional sound.

"This activation occurred in parts of the brain that you would use to contort your face into a grin or a chuckle," the professor says.

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

Laughter, says the expert, can be contagious.

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

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

When it comes to Christmas cracker puns, she says, the feel-good effect is more probable to be triggered not by the gag in itself, but from the response to it.

"The laughter is key. The joke is the dreadful Christmas cracker pun, and it's just a pretext to laugh together."

The Quest for the Ideal Festive Pun

Will we ever discover the ultimate joke?

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

Years ago, a professor established a scientific search for the planet's most humorous joke.

Over 40,000 gags submitted, with ratings provided by hundreds of thousands of people around the world, he has a clearer understanding than most as to what succeeds and what fails.

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

"They must also be bad gags, jokes that cause us to moan," he continues.

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

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

"What's interesting about the holiday cracker jokes is that none of us considers them funny.

"That's a shared experience around the table and I think it's wonderful."