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

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

This describes a humor-evaluation session with a company that produces products for social events. Its repertoire features festive crackers.

The company's founder smiles, nearly apologetically at the joke. But the pun has been selected and will feature in future crackers.

"The success is gauged by the gag by the volume of groans and the loudness of the groans at the table," she explains.

The key to a great holiday cracker joke is not the identical as a good joke per se. It is entirely about the setting - in this instance, the shared amusement of the holiday meal with grandparents, kids and potentially neighbours.

"You want the joke to be something that brings the child together with the 80-year-old," she adds.

The Science Of Communal Amusement

Coming together to experience communal laughter is not only nothing new, experts say, it is likely to be pre-human.

"So when you are laughing with others around the holiday table you are engaging in what's almost certainly a really primordial mammalian social sound," explains a professor.

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

Scientists have discovered that a lack of such social exchanges can significantly harm mental and physical health.

"The people you converse with, and share laughter with, it results in enhanced amounts of 'happy chemical' release," the professor adds.

Endorphins are the body's "happy chemicals" and are released both to reduce stress and pain and in response to pleasurable activities, such as chuckling with friends over a particularly awful Christmas cracker gag.

"You're not just laughing at a silly pun with a Christmas cracker," the expert states. "You are in fact performing a lot of the really important task of making, maintaining the connections you have with those you love."

What Happens Inside the Mind?

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

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

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which parts of the brain are working harder, scientists have been able to chart the regions that receive more blood flow.

The research entails scanning the minds of volunteer participants and then exposing them to a database of humorous words, accompanied by either a non-emotional sound, or pre-recorded chuckles.

"During the study we got a really interesting pattern of activation," says the professor.

A gag stimulates not just the parts of the mind responsible for auditory processing and understanding language, but also neural regions associated with both planning and starting motion and those linked to vision and recall.

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

The Infectious Nature of Laughter

Scientists discovered that when a funny phrase is combined with laughter there is a stronger response in the brain than the identical word when accompanied by a neutral sound.

"This activation occurred in parts of the brain that you would use to move your expression into a smile or a laugh," she explains.

It means people are not just responding to funny jokes, they are reacting to the amusement that follows them.

Amusement, says the expert, can be infectious.

So what does this mean for the laughter found around a Christmas gathering?

"You laugh harder when you know people," she notes, "and you laugh more when you like them or care for them."

When it comes to Christmas cracker puns, she says, the positive effect is more likely to be triggered not by the gag itself, but from the reaction to it.

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

The Quest for the Ideal Festive Pun

Will we ever find the perfect gag?

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

In 2001, a professor established a scientific search for the world's funniest joke.

More than tens of thousands of gags submitted, with scores lodged by 350,000 people around the world, he has a better idea than most as to what works and what does not.

The perfect festive cracker pun needs to be short, he explains.

"They must also need to be poor jokes, jokes that cause us to moan," he adds.

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

"This is because if no-one laughs – it's the gag's fault, not your own.

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

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