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

This one-liner is greeted with moans that echo through a warehouse in London.

This describes a humor-evaluation session with a company that produces supplies for gatherings. Its repertoire features Christmas crackers.

The firm's founder smiles, almost apologetically at the gag. But the pun has made the cut and will appear in future crackers.

"You measure the gag by the number of moans and the intensity of the groans around the table," the founder explains.

The key to a great Christmas cracker joke is not the same as a stand-up joke per se. It is entirely about the context - in this case, the communal laughter of the Christmas meal with elders, kids and possibly friends.

"You want the gag to be something that unites the eight-year-old in harmony with the 80-year-old," she states.

The Science Of Communal Amusement

Gathering to enjoy communal amusement is not only nothing new, experts say, it is probably to be pre-human.

"So when you are chuckling with people around the holiday table you are engaging in what's very likely a truly primordial mammalian play sound," says a professor.

Shared amusement, she says, aids in forge and strengthen social connections between people.

Scientists have discovered that a lack of these interactions can significantly damage mental and physical health.

"The people you talk to, and share laughter with, it results in enhanced amounts of 'happy chemical' uptake," she adds.

Endorphins are the brain's "feel-good compounds" and are produced both to reduce tension and discomfort and in response to pleasurable activities, such as laughing with friends over a truly terrible festive cracker gag.

"You're not just chuckling at a foolish joke with a Christmas cracker," she states. "You are actually doing a lot of the really vital task of building, preserving the connections you have with the people you love."

Which Happens Inside the Mind?

But what is actually happening within the mind when we hear a gag?

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

Using functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which parts of the brain are working harder, scientists have been able to chart the regions that receive more blood.

Testing involves imaging the minds of healthy subjects and then subjecting them to a collection of funny words, paired with either a neutral sound, or pre-recorded laughter.

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

A gag stimulates not just the parts of the brain responsible for hearing and interpreting speech, but also brain regions involved in both preparation and starting movement and those involved in sight and memory.

Combine these elements as a whole, and people listening to a pun have a sophisticated series of neural responses that underpin the laughter we experience.

The Infectious Nature of Laughter

Researchers found that when a humorous word is combined with chuckles there is a greater reaction in the brain than the same word when accompanied by a non-emotional sound.

"This was in parts of the mind that you would use to move your face into a smile or a laugh," the professor explains.

It indicates people are not just reacting to funny jokes, they are reacting to the laughter that follows them.

Laughter, according to the expert, can be infectious.

So what does this mean for the laughter heard around a Christmas table?

"You laugh more when you know people," she says, "and you laugh more when you like them or love them."

When it comes to festive cracker puns, she says, the feel-good factor is more likely to be caused not by the gag itself, but from the reaction to it.

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

The Search for the Ideal Festive Pun

Is it possible to discover the perfect joke?

Likely not, but that has not stopped researchers from attempting to.

Years ago, a professor established a research project for the world's funniest joke.

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

The perfect Christmas cracker pun must be brief, he says.

"But they also need to be bad jokes, puns that cause us to moan," he adds.

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

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

"What's interesting about the Christmas cracker puns is that none of us find them humorous.

"That's a common experience at the gathering and I think it's wonderful."