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

This quip is greeted with moans that resonate through a storage facility in London.

We're at a humor-evaluation session with a company that produces products for gatherings. Its repertoire features festive crackers.

The firm's owner smiles, nearly sheepishly at the joke. But the pun has been selected and will appear in future crackers.

"The success is gauged by the gag by the number of moans and the loudness of the groans around the table," she explains.

The secret to a great Christmas cracker pun is not the same as a stand-up gag per se. It is entirely about the setting - in this case, the communal amusement of the Christmas meal with elders, children and potentially neighbours.

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

The Neuroscience Behind Shared Amusement

Coming together to enjoy communal laughter is not only nothing new, scientists say, it is probably to be pre-human.

"Therefore when you are chuckling with people at the holiday dinner you are dropping into what's very likely a truly primordial mammalian social sound," says a professor.

Communal amusement, she explains, helps make and maintain social bonds between individuals.

Scientists have discovered that a absence of such interactions can significantly harm both psychological and bodily health.

"The people you converse with, and laugh with, it results in enhanced amounts of endorphin release," she continues.

Endorphins are the body's "happy chemicals" and are produced both to alleviate stress and pain and in reaction to pleasurable activities, such as laughing with friends over a truly awful Christmas cracker joke.

"You're not just laughing at a foolish joke with a holiday cracker," she states. "You are actually performing a lot of the really important work of building, preserving the connections you have with those you care about."

Which Occurs In the Brain?

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

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

Using brain scanning technology, a type of neural imager which shows which parts of the mind are more active, scientists have been able to chart the areas that get more blood flow.

Testing entails imaging the minds of volunteer participants and then exposing them to a database of funny words, accompanied by either a neutral sound, or recorded chuckles.

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

A joke stimulates not just the areas of the mind responsible for hearing and interpreting language, but also neural areas involved in both planning and initiating motion and those linked to sight and memory.

Put all of this as a whole, and individuals hearing a joke have a complex series of neural responses that support the laughter we experience.

The Infectious Nature of Chuckles

Researchers discovered that when a humorous word is combined with laughter there is a stronger response in the brain than the identical phrase when followed by a non-emotional sound.

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

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

Laughter, according to the professor, can be infectious.

So what does this imply for the laughter found at a holiday table?

"You laugh harder when you are familiar with people," she notes, "and laughter increases further when you like them or love them."

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

"The laughter is key. The joke is the terrible holiday cracker pun, and it's just a reason to laugh together."

The Search for the Perfect Cracker Joke

Will we ever find the ultimate gag?

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

Years ago, a psychologist set up a research search for the world's funniest joke.

More than 40,000 jokes later, with scores provided by hundreds of thousands of people around the world, he has a clearer understanding than many as to what succeeds and what fails.

The ideal festive cracker joke must be short, he explains.

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

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

"This is because if nobody finds it funny – it's the joke's shortcoming, not yours.

"The fascinating part about the Christmas cracker puns is that not one person find them funny.

"That's a shared experience at the gathering and I believe it's wonderful."