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

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

We're at a humor-evaluation session with a firm that makes products for social events. Its repertoire includes Christmas crackers.

The company's founder smiles, nearly sheepishly at the gag. But the pun has been selected and will appear in future crackers.

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

The secret to a good Christmas cracker pun is not the same as a good gag per se. It is all about the setting - in this case, the shared laughter of the Christmas meal with elders, children and potentially neighbours.

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

The Neuroscience Of Shared Amusement

Gathering to enjoy communal amusement is not only nothing new, experts say, it is likely 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 primordial mammalian social vocalisation," says a professor.

Shared laughter, she explains, aids in make and maintain social bonds between individuals.

Researchers have found that a lack of such interactions can seriously damage mental and physical well-being.

"Those you talk to, and share laughter with, it leads to enhanced amounts of 'happy chemical' uptake," she adds.

Endorphins are the body's "happy chemicals" and are released both to alleviate tension and discomfort and in response to enjoyable activities, such as chuckling with friends over a particularly terrible festive cracker joke.

"It's not simply chuckling at a foolish pun with a holiday cracker," the expert states. "You are actually performing a lot of the really vital work of building, preserving the connections you have with the people you love."

What Occurs Inside the Mind?

But what is truly taking place within the brain when we listen to a joke?

An awful lot occurs in response to humour, it turns out.

Using brain scanning technology, a kind of neural imager which indicates which parts of the brain are more active, scientists have been able to chart the areas that receive more blood.

Testing entails scanning the brains of volunteer subjects and then subjecting them to a collection of funny words, accompanied by either a neutral sound, or recorded chuckles.

"In the scanner we observed a really fascinating pattern of activation," notes the professor.

A joke activates not just the parts of the brain responsible for hearing and interpreting speech, but also neural regions involved in both preparation and starting motion and those involved in sight and memory.

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

The Infectious Power of Laughter

Scientists found that when a funny phrase is combined with laughter there is a greater reaction in the mind than the identical phrase when followed by a neutral sound.

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

It means people are not just responding to funny 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 found around a Christmas table?

"People laugh harder when you know others," she says, "and laughter increases more when you like them or care for them."

When it comes to Christmas cracker puns, she explains, the feel-good factor is more likely to be triggered not by the joke in itself, but from the reaction to it.

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

The Quest for the Ideal Cracker Joke

Will we ever find the perfect joke?

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

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

Over tens of thousands of gags later, with scores provided by 350,000 participants globally, he has a clearer understanding than most as to what works and what fails.

The perfect Christmas cracker joke needs to be short, he says.

"But they also be bad gags, jokes that make us groan," he adds.

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

"The reason is that if nobody finds it funny – it's the joke's fault, not yours.

"The fascinating part about the holiday cracker jokes is that none of us find them humorous.

"That's a shared experience around the table and I think it's lovely."