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

This joke is met by moans that echo through a warehouse in London.

This describes a joke-testing meeting with a company that makes supplies for gatherings. Its repertoire includes festive crackers.

The firm's owner grins, nearly apologetically at the joke. But the joke has made the cut and will appear in future crackers.

"You measure the gag by the volume of groans and the loudness of the groans around the table," the founder explains.

The secret to a good holiday cracker joke is not the same as a stand-up gag per se. It is entirely about the context - in this case, the communal laughter of the holiday meal with grandparents, children and potentially friends.

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

The Neuroscience Of Shared Laughter

Coming together to enjoy shared amusement is not only ancient, experts say, it is likely to be older than humanity.

"So when you are chuckling with others at the holiday dinner you are engaging in what's almost certainly a really primordial mammal social vocalisation," explains a neuroscience expert.

Communal laughter, she says, aids in forge and strengthen social bonds between individuals.

Scientists have discovered that a absence of such social exchanges can significantly damage mental and physical health.

"The people you talk to, and laugh with, it results in enhanced amounts of endorphin release," the professor continues.

These natural chemicals are the brain's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to enjoyable experiences, such as chuckling with loved ones over a truly awful festive cracker gag.

"You're not just laughing at a silly pun with a holiday cracker," the expert says. "You are actually performing a lot of the truly important task of making, maintaining the connections you have with the people you love."

What Happens Inside the Mind?

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

An awful lot 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 working harder, researchers have been able to map the areas that receive more blood.

Testing involves scanning the brains of healthy subjects and then exposing them to a collection of funny words, paired with either a neutral sound, or recorded laughter.

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

A joke activates not just the parts of the brain responsible for hearing and interpreting language, but also neural areas involved in both planning and starting motion and those involved in vision and memory.

Combine these elements together, and individuals hearing a pun have a complex set of neural responses that support the laughter we hear.

The Contagious Power of Chuckles

Scientists discovered that when a funny word is combined with laughter there is a stronger response in the brain than the same phrase when accompanied by a neutral sound.

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

It indicates we are not just reacting to humorous words, they are reacting to the amusement that accompanies them.

Laughter, says the expert, can be contagious.

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

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

When it comes to festive cracker jokes, she says, the positive effect is more likely to be triggered not by the joke itself, but from the response to it.

"The laughter is key. The gag is the terrible holiday cracker joke, and it's just a reason to chuckle together."

The Search for the Perfect Cracker Joke

Will we ever find the ultimate joke?

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

Years ago, a psychologist set up a scientific search for the world's most humorous gag.

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

The perfect festive cracker pun needs to be brief, he says.

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

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

"The reason is that if no-one finds it funny – it's the joke's shortcoming, not your own.

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

"That's a common experience at the table and I think it's lovely."