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"How much did Father Christmas's sleigh cost? Zero, it was on the house."

This joke is met by groans that resonate through a warehouse in London.

We're at a humor-evaluation meeting with a firm that produces products for gatherings. Its catalogue includes festive crackers.

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

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

The secret to a great Christmas cracker pun is not the identical as a stand-up joke in itself. It is entirely about the setting - in this case, the shared laughter of the Christmas meal with grandparents, kids and potentially friends.

"You want the gag to be something that brings the child together with the 80-year-old," she states.

The Science Behind Shared Laughter

Coming together to experience shared laughter is not only ancient, scientists argue, it is likely to be older than humanity.

"Therefore when you are laughing with people around the holiday table you are engaging in what's very likely a really ancient mammal play sound," explains a professor.

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

Researchers have discovered that a lack of these social exchanges can seriously damage mental and physical health.

"Those you talk to, and share laughter with, it leads to increased levels of 'happy chemical' uptake," she continues.

Endorphins are the brain's "feel-good compounds" and are produced both to reduce stress and pain and in response to enjoyable activities, such as laughing with friends over a truly awful Christmas cracker joke.

"You're not just laughing at a silly pun with a holiday cracker," the expert states. "You are in fact performing a lot of the really important task of making, maintaining the social bonds you have with those you love."

Which Occurs In the Mind?

But what is truly happening inside the mind when we hear a gag?

An awful lot occurs in reaction to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which shows which parts of the mind are working harder, researchers have been able to map the areas that receive more blood flow.

Testing involves imaging the brains of volunteer subjects and then exposing them to a database of funny words, accompanied by either a non-emotional sound, or recorded laughter.

"In the scanner we observed a very fascinating activation pattern of neural activity," says the professor.

A joke stimulates not just the parts of the brain responsible for hearing and interpreting language, but also brain areas involved in both preparation and initiating motion and those involved in vision and memory.

Put these elements as a whole, and people hearing a pun have a sophisticated series of neural responses that underpin the laughter we experience.

The Infectious Power of Laughter

Researchers found that when a funny phrase is combined with laughter there is a stronger response in the mind than the same phrase when followed by a non-emotional sound.

"This was in areas of the mind that you would use to contort your expression into a smile or a chuckle," the professor says.

It indicates we are not just responding to humorous words, they are reacting to the laughter that accompanies them.

Laughter, says the expert, can be infectious.

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

"You laugh more when you are familiar with others," she notes, "and laughter increases more when you like 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 itself, but from the reaction to it.

"The laughter is key. The joke is the dreadful Christmas cracker joke, and it's just a pretext to chuckle together."

The Quest for the Perfect Cracker Joke

Will we ever find the ultimate joke?

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

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

More than 40,000 jokes later, with ratings provided by 350,000 people around the world, he has a clearer idea than most as to what succeeds and what does not.

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

"They must also need to be bad jokes, puns that cause us to moan," he adds.

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

"This is because if nobody laughs – it's the gag's fault, not your own.

"What's interesting about the Christmas cracker jokes is that none of us find them humorous.

"It creates a shared experience at the gathering and I believe it's wonderful."