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"What was the price did Santa's sled cost? Nothing, it was on the house."

This one-liner is greeted with groans that resonate through a warehouse in London.

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

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

"You measure the gag by the volume of moans and the intensity of the groans around the table," the founder explains.

The secret to a great Christmas cracker pun is not the same as a good joke per se. It is all about the context - in this instance, the communal laughter of the holiday dinner table with elders, kids and possibly neighbours.

"You want the gag to be something that brings the child together with the grandparent," she states.

The Science Behind Communal Amusement

Gathering to enjoy communal laughter is not only ancient, experts say, it is probably to be older than humanity.

"So when you are chuckling with people at the Christmas table you are engaging in what's almost certainly a really primordial mammalian play sound," explains a neuroscience expert.

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

Researchers have found that a lack of these interactions can significantly harm mental and physical health.

"The people you converse with, and share laughter with, it leads to increased levels of endorphin release," the professor continues.

These natural chemicals are the brain's "feel-good compounds" 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 festive cracker joke.

"You're not just laughing at a foolish pun with a Christmas cracker," she says. "You are actually performing a lot of the really vital work of making, maintaining the social bonds you have with the people you care about."

What Happens Inside the Mind?

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

An awful lot happens in response to comedy, it transpires.

Employing functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which areas of the mind are more active, researchers have been able to map the regions that get more blood.

Testing entails imaging the minds of volunteer participants and then subjecting them to a collection of humorous words, paired with either a non-emotional sound, or pre-recorded laughter.

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

A joke stimulates not just the areas of the mind responsible for hearing and interpreting speech, but also neural areas associated with both preparation and initiating movement and those involved in vision and recall.

Put these elements as a whole, and people hearing a pun have a sophisticated set of brain responses that support the laughter we hear.

The Infectious Nature of Chuckles

Scientists discovered that when a funny word is combined with chuckles there is a greater response in the brain than the identical word when followed by a neutral sound.

"This activation occurred in parts of the mind that you would use to move your expression into a smile or a laugh," the professor says.

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

Laughter, says the expert, can be contagious.

So what does this imply for the chuckles found around a holiday gathering?

"You laugh harder when you know people," she says, "and you laugh more 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 gag in itself, but from the response to it.

"It's the laughter. The gag is the terrible Christmas cracker joke, and it's just a pretext to chuckle as a group."

The Quest for the Ideal Cracker Joke

Is it possible to discover the perfect gag?

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

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

More than 40,000 gags submitted, with scores lodged by 350,000 participants globally, he has a better idea than many as to what succeeds and what does not.

The perfect festive cracker pun must be brief, he explains.

"They must also be bad gags, jokes that cause us to groan," he adds.

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

"This is because if no-one laughs – it's the gag's shortcoming, not yours.

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

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