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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 groans that echo through a storage facility in the capital.

We're at a humor-evaluation session with a company that makes products for social events. Its catalogue includes festive crackers.

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

"The success is gauged by the joke by the number of groans and the intensity of the groans at the table," the founder says.

The key to a great holiday cracker pun is not the identical as a stand-up gag in itself. It is entirely about the setting - in this instance, the communal laughter of the Christmas dinner table with elders, kids and potentially friends.

"The goal is for the joke to be a thing that unites the child in harmony with the 80-year-old," she adds.

The Neuroscience Of Shared Amusement

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

"So when you are laughing with people at the Christmas table you are dropping into what's almost certainly a truly ancient mammalian play vocalisation," explains a professor.

Communal laughter, she explains, aids in make and maintain social connections between people.

Researchers have discovered that a lack of these social exchanges can seriously damage both psychological and bodily well-being.

"The people you talk to, and share laughter with, it results in increased levels of 'happy chemical' release," she continues.

Endorphins are the body's "happy chemicals" and are released both to reduce stress and pain and in response to enjoyable activities, such as chuckling with loved ones over a truly terrible Christmas cracker joke.

"You're not just laughing at a foolish pun with a holiday cracker," she says. "You are actually doing a lot of the really vital task of building, preserving the social bonds you have with the people you love."

Which Occurs In the Brain?

But what is truly taking place within the mind when we hear a joke?

A tremendous amount happens in reaction to humour, it turns out.

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

The research entails imaging the brains of healthy subjects and then exposing them to a database of humorous phrases, paired with either a non-emotional sound, or pre-recorded chuckles.

"During the study we observed a really fascinating pattern of activation," notes the professor.

A joke stimulates not just the areas of the brain responsible for auditory processing and understanding speech, but also neural areas involved in both preparation and starting movement and those linked to sight and memory.

Put all of this as a whole, and people hearing a pun have a sophisticated series of neural responses that underpin the laughter we hear.

The Contagious Nature of Laughter

Scientists discovered that when a funny phrase is paired with laughter there is a greater reaction in the mind than the same word when accompanied by a neutral sound.

"This was in parts of the mind that you would employ to contort your face into a smile or a laugh," she says.

It means people are not just responding to humorous words, they are reacting to the laughter that follows them.

Amusement, says the expert, can be infectious.

So what does this mean for the laughter heard at a holiday table?

"People laugh harder when you know people," she notes, "and you laugh more when you are fond of them or care for them."

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

"The laughter is key. The joke is the terrible Christmas cracker pun, and it's just a pretext 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 prevented researchers from attempting to.

Years ago, a professor set up a scientific project for the planet's most humorous gag.

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

The perfect Christmas cracker joke needs to be brief, he explains.

"They must also be bad gags, puns that make us groan," he continues.

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

"The reason is that 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 humorous.

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