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"What was the price did Santa's sleigh cost? Zero, it was on the house."

This one-liner is met by groans that echo through a storage facility in London.

We're at a joke-testing meeting with a firm that makes products for social events. Its repertoire features Christmas crackers.

The firm's founder smiles, almost apologetically at the gag. But the pun has made the cut and will appear in upcoming crackers.

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

The secret to a good Christmas cracker joke is not the same as a stand-up joke per se. It is all about the context - in this case, the shared amusement of the Christmas dinner table with grandparents, children and possibly neighbours.

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

The Neuroscience Of Shared Amusement

Coming together to experience shared laughter is not only ancient, scientists argue, it is probably to be pre-human.

"Therefore when you are chuckling with people at the Christmas table you are engaging in what's almost certainly a really ancient mammalian play vocalisation," says a neuroscience expert.

Shared amusement, she says, aids in make and maintain social connections between individuals.

Researchers have discovered that a lack of such interactions can significantly damage mental and physical well-being.

"The people you talk to, and laugh with, it leads to enhanced amounts of 'happy chemical' release," she adds.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce stress and pain and in response to enjoyable experiences, such as laughing with friends over a particularly terrible festive cracker joke.

"It's not simply chuckling at a foolish joke with a holiday cracker," she states. "You are actually doing a lot of the truly important work of building, preserving the social bonds you have with those you care about."

What Occurs In the Mind?

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

An awful lot occurs in response to humour, it transpires.

Using brain scanning technology, a type of brain scanner which indicates which parts of the mind are working harder, researchers have been able to map the areas that get more blood.

Testing involves imaging the minds of volunteer subjects and then subjecting them to a database of humorous words, paired with either a neutral sound, or pre-recorded laughter.

"During the study we got a very fascinating pattern of neural activity," says the neuroscientist.

A joke activates not just the areas of the brain responsible for hearing and interpreting speech, but also brain areas associated with both planning and starting motion and those linked to vision and recall.

Combine these elements as a whole, and people listening to a joke have a complex set of neural responses that support the laughter we hear.

The Infectious Nature of Chuckles

Researchers found that when a humorous word is paired with laughter there is a stronger response in the mind than the same phrase when accompanied by a non-emotional sound.

"This was in areas of the brain that you would employ to contort your face into a grin or a laugh," the professor explains.

It indicates we are not just responding to funny words, they are responding to the amusement that follows them.

Amusement, according to the professor, can be infectious.

So what does this imply for the chuckles heard around a Christmas table?

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

When it comes to Christmas cracker jokes, she says, the positive factor is more likely to be caused not by the joke in itself, but from the reaction to it.

"It's the laughter. The joke is the terrible holiday cracker pun, and it's just a pretext to laugh as a group."

The Search for the Ideal Cracker Joke

Is it possible to find the ultimate gag?

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

Years ago, a professor established a research project for the world's most humorous gag.

Over tens of thousands of gags later, with ratings lodged by 350,000 people around the world, he has a clearer understanding than most as to what works and what fails.

The ideal Christmas cracker pun needs to be short, he says.

"They must also be poor jokes, jokes that make us moan," he continues.

The more "terrible" the joke, he says the better.

"The reason is that if nobody laughs – it's the gag's fault, not yours.

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

"That's a shared moment at the table and I believe it's lovely."