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

This one-liner is met by groans that resonate through a storage facility in London.

We're at a humor-evaluation meeting with a firm that produces supplies for gatherings. Its catalogue includes Christmas crackers.

The company's owner grins, nearly sheepishly at the joke. But the joke has made the cut and will appear in upcoming crackers.

"The success is gauged by the joke by the number of groans and the intensity of the groans around the table," the founder explains.

The key to a good holiday cracker pun is not the same as a stand-up joke per se. It is all about the context - in this case, the communal amusement of the Christmas dinner table with elders, kids and possibly friends.

"The goal is for the gag to be something that unites the child together with the grandparent," she states.

The Science Behind Shared Laughter

Coming together to enjoy shared laughter is not only ancient, experts argue, it is probably to be older than humanity.

"So when you are chuckling with others around the holiday dinner you are engaging in what's very likely a really ancient mammal play vocalisation," explains a professor.

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

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

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

Endorphins are the body's "happy chemicals" and are produced both to reduce stress and pain and in reaction to enjoyable activities, such as laughing with loved ones over a truly awful Christmas cracker gag.

"You're not just laughing at a silly joke with a Christmas cracker," she says. "You are in fact performing a lot of the really important task of building, preserving the connections you have with those you love."

Which Happens Inside the Brain?

But what is truly taking place inside the brain when we hear a gag?

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

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which parts of the brain are working harder, scientists have been able to map the areas that get more blood.

The research involves imaging the brains of healthy subjects and then exposing them to a database of funny phrases, paired with either a neutral sound, or pre-recorded laughter.

"During the study we got a very fascinating pattern of activation," notes the professor.

A joke activates not just the parts of the brain in charge of auditory processing and interpreting speech, but also neural regions associated with both planning and starting motion and those involved in sight and recall.

Combine all of this together, and people hearing a pun have a sophisticated series of neural responses that support the laughter we hear.

The Infectious Power of Laughter

Researchers found that when a humorous word is paired with chuckles there is a stronger response in the brain than the identical phrase when accompanied by a neutral sound.

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

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

Laughter, says the expert, can be contagious.

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

"You laugh more when you know others," she notes, "and laughter increases further when you like them or love them."

When it comes to festive cracker jokes, she says, the positive factor is more probable to be triggered not by the gag in itself, but from the reaction to it.

"It's the laughter. The gag is the dreadful holiday cracker pun, and it's just a pretext to laugh together."

The Quest for the Perfect Festive Pun

Will we ever discover the ultimate joke?

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

Years ago, a psychologist set up a research project for the world's funniest gag.

More than tens of thousands of jokes submitted, with ratings lodged by hundreds of thousands of people globally, he has a better understanding than most as to what works and what does not.

The perfect festive cracker joke must be short, he says.

"They must also be bad jokes, jokes that make us groan," he continues.

The increasingly "terrible" the gag, he states the better.

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

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

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