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

This one-liner is met by moans that echo through a storage facility in London.

We're at a joke-testing meeting with a company that produces supplies for gatherings. Its repertoire features festive crackers.

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

"The success is gauged by the gag by the number of moans and the intensity of the groans around the table," the founder says.

The secret to a great Christmas cracker pun is not the same as a stand-up gag per se. It is all about the setting - in this case, the communal amusement of the Christmas meal with grandparents, kids and possibly friends.

"You want the gag to be something that unites the eight-year-old in harmony with the grandparent," she states.

The Neuroscience Behind Communal Laughter

Coming together to experience shared amusement is not only nothing new, experts say, it is probably to be older than humanity.

"Therefore when you are laughing with people at the holiday dinner you are engaging in what's very likely a really ancient mammal play sound," explains a professor.

Shared laughter, she says, helps forge and strengthen social connections between people.

Researchers have discovered that a absence of these interactions can significantly damage both psychological and bodily well-being.

"The people you talk to, and share laughter with, it leads to increased levels of endorphin uptake," the professor continues.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to enjoyable activities, such as laughing with loved ones over a particularly terrible festive cracker joke.

"It's not simply laughing at a silly joke with a holiday cracker," the expert states. "You are actually performing a lot of the truly vital task of building, preserving the connections you have with the people you love."

What Happens Inside the Brain?

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

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which areas of the brain are more active, scientists have been able to map the regions that receive more blood.

The research entails scanning the minds of volunteer subjects and then subjecting them to a collection of funny words, accompanied by either a neutral sound, or recorded laughter.

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

A joke activates not just the areas of the mind responsible for auditory processing and understanding speech, but also neural regions involved in both preparation and starting motion and those linked to sight and memory.

Put these elements together, and people listening to a pun have a complex set of neural reactions that support the amusement we hear.

The Infectious Nature of Chuckles

Scientists discovered that when a humorous phrase is combined with laughter there is a greater response in the brain than the identical phrase when accompanied by a neutral sound.

"This was in parts of the mind that you would use to move your expression into a grin or a chuckle," she says.

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

Amusement, says the professor, can be contagious.

So what does this imply for the chuckles heard around a holiday gathering?

"People laugh more when you are familiar with people," she notes, "and laughter increases more when you are fond of 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 joke in itself, but from the reaction to it.

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

The Search for the Ideal Festive Pun

Is it possible to discover the ultimate gag?

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

Years ago, a professor established a research project for the world's most humorous gag.

More than tens of thousands of gags later, with scores provided by 350,000 people globally, he has a clearer understanding than many as to what succeeds and what fails.

The perfect festive cracker pun must be brief, he explains.

"They must also be poor gags, jokes that cause us to moan," he adds.

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

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

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

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