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

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

We're at a humor-evaluation session with a company that produces supplies for social events. Its catalogue includes Christmas crackers.

The company's owner smiles, nearly sheepishly at the joke. But the pun has made the cut and will appear in future crackers.

"The success is gauged by the gag by the volume of moans and the intensity of the groans at the table," she says.

The key to a great holiday cracker pun is not the same as a good joke in itself. It is all about the setting - in this case, the communal amusement of the Christmas dinner table with elders, children and possibly neighbours.

"The goal is for the gag to be a thing that brings the child together with the 80-year-old," she states.

The Neuroscience Of Communal Amusement

Coming together to enjoy communal laughter is not only ancient, scientists say, it is likely to be older than humanity.

"Therefore when you are chuckling with others around the holiday dinner you are engaging in what's almost certainly a really ancient mammal social vocalisation," says a professor.

Shared amusement, she says, helps make and maintain social bonds between people.

Scientists have found that a lack of such social exchanges can seriously damage both psychological and bodily health.

"The people you talk to, and share laughter with, it leads to increased amounts of endorphin release," she continues.

Endorphins are the brain'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 truly awful festive cracker joke.

"It's not simply chuckling at a silly pun with a Christmas cracker," she says. "You are actually performing a lot of the truly vital task of making, maintaining the social bonds you have with the people you care about."

What Happens In the Mind?

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

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

Employing brain scanning technology, a kind of brain scanner which shows which areas of the mind are working harder, researchers have been able to map the areas that receive more blood flow.

Testing entails scanning the minds of volunteer subjects and then exposing them to a collection of funny words, paired with either a neutral sound, or recorded laughter.

"During the study we observed a very interesting pattern of neural activity," notes the professor.

A gag stimulates not just the areas of the brain in charge of hearing and understanding language, but also brain areas involved in both planning and starting motion and those involved in sight and memory.

Combine all of this together, and individuals listening to a joke have a sophisticated series of neural responses that support the laughter we experience.

The Infectious Nature of Chuckles

Scientists found that when a funny phrase is combined with laughter there is a stronger reaction in the brain than the same word when followed by a non-emotional sound.

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

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

Laughter, says the expert, can be infectious.

So what does this imply for the chuckles heard at a holiday gathering?

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

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

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

The Quest for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

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

Over 40,000 jokes later, with scores provided by hundreds of thousands of participants around the world, he has a better understanding than many as to what succeeds and what does not.

The perfect festive cracker pun needs to be brief, he says.

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

The increasingly "awful" the gag, he states the more effective.

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

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

"It creates a common experience around the table and I believe it's lovely."