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

This joke is greeted with groans that echo through a warehouse in London.

We're at a joke-testing session with a firm that produces supplies for social events. Its repertoire includes festive crackers.

The firm's owner grins, almost apologetically at the gag. But the joke has made the cut and will feature in future crackers.

"You measure the gag by the volume of groans and the loudness of the groans around the table," she says.

The key to a good Christmas cracker joke is not the same as a stand-up joke in itself. It is entirely about the context - in this instance, the communal laughter of the Christmas dinner table with elders, children and possibly friends.

"You want the gag to be a thing that unites the child in harmony with the grandparent," she adds.

The Neuroscience Of Communal Amusement

Coming together to enjoy shared amusement is not only nothing new, scientists say, it is probably to be older than humanity.

"So when you are laughing with people around the Christmas table you are engaging in what's very likely a truly ancient mammalian play vocalisation," explains a professor.

Shared amusement, she explains, helps forge and strengthen social connections between individuals.

Researchers have discovered that a absence of such social exchanges can significantly damage mental and physical health.

"The people you talk to, and share laughter with, it leads to enhanced amounts of endorphin release," the professor continues.

Endorphins are the brain's "happy chemicals" and are produced both to alleviate stress and pain and in response to pleasurable activities, such as chuckling with friends over a truly awful festive cracker gag.

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

Which Occurs Inside the Brain?

But what is actually happening within the mind when we hear a joke?

An awful lot happens in reaction to comedy, it turns out.

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

Testing entails scanning the brains of volunteer participants and then subjecting them to a database of funny phrases, accompanied by either a non-emotional sound, or pre-recorded chuckles.

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

A gag stimulates not just the parts of the mind responsible for hearing and understanding speech, but also neural areas associated with both planning and starting motion and those linked to sight and memory.

Combine these elements together, and individuals listening to a joke have a complex set of brain reactions that underpin the amusement we experience.

The Infectious Nature of Laughter

Researchers found that when a funny phrase is paired with laughter there is a stronger reaction in the brain than the same phrase when accompanied by a non-emotional sound.

"This was in parts of the mind that you would use to contort your expression into a smile or a laugh," she explains.

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

Amusement, says the professor, can be infectious.

So what does this imply for the chuckles found at a Christmas gathering?

"People laugh more when you are familiar with others," she says, "and laughter increases further when you are fond of them or care for them."

When it comes to Christmas cracker jokes, she says, the feel-good factor is more probable to be caused not by the joke 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 reason to chuckle as a group."

The Quest for the Ideal Festive Pun

Is it possible to find the perfect gag?

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

Years ago, a psychologist set up a scientific search for the planet's funniest joke.

More than 40,000 gags later, with scores provided by 350,000 participants globally, he has a clearer understanding than many as to what works and what fails.

The ideal Christmas cracker joke must be brief, he explains.

"They must also be bad jokes, puns that make us groan," he adds.

The more "awful" the joke, he states the more effective.

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

"The fascinating part about the holiday cracker puns is that not one person considers them humorous.

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