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

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

We're at a joke-testing session with a company that makes supplies for social events. Its catalogue features Christmas crackers.

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

"You measure the gag by the number of groans and the loudness of the groans at the table," she explains.

The key to a good holiday cracker joke is not the same as a stand-up gag 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.

"You want the joke to be a thing that brings the eight-year-old together with the grandparent," she adds.

The Science Behind Communal Amusement

Gathering to experience shared laughter is not only nothing new, scientists say, it is probably to be pre-human.

"So when you are chuckling with people at the holiday dinner you are dropping into what's almost certainly a truly ancient mammalian social vocalisation," explains a professor.

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

Researchers have discovered that a lack of these social exchanges can significantly damage mental and physical well-being.

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

Endorphins are the body's "happy chemicals" and are released both to reduce tension and discomfort and in reaction to pleasurable experiences, such as laughing with loved ones over a truly terrible festive cracker gag.

"It's not simply chuckling at a silly joke with a holiday cracker," the expert states. "You are in fact performing a lot of the truly vital task of building, preserving the social bonds you have with the people you care about."

What Occurs In the Brain?

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

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

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which areas of the mind are more active, scientists have been able to chart the areas that receive more blood.

Testing involves imaging the minds of healthy participants and then subjecting them to a collection of humorous phrases, paired with either a neutral sound, or pre-recorded chuckles.

"In the scanner we observed a very fascinating activation pattern of activation," says the neuroscientist.

A gag stimulates not just the parts of the mind in charge of auditory processing and understanding language, but also brain areas associated with both planning and initiating movement and those involved in vision and recall.

Put these elements as a whole, and people listening to a pun have a sophisticated series of brain reactions that support the amusement we hear.

The Contagious Nature of Chuckles

Researchers found that when a funny phrase is paired with laughter there is a greater response in the mind than the identical word when followed by a neutral sound.

"This activation occurred in parts of the brain that you would employ to move your expression into a grin or a laugh," she says.

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

Laughter, according to the expert, can be contagious.

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

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

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

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

The Search for the Ideal Cracker Joke

Will we ever find the ultimate joke?

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

In 2001, a psychologist set up a research search for the planet's funniest gag.

More than tens of thousands of jokes later, with ratings provided by 350,000 participants around the world, he has a clearer idea than most as to what succeeds and what fails.

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

"They must also need to be poor gags, puns that make us groan," he continues.

The increasingly "awful" the gag, he says the more effective.

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

"What's interesting about the holiday cracker jokes is that not one person find them humorous.

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