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"How much did Santa's sled cost? Zero, it was on the house."

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

This describes a humor-evaluation meeting with a company that produces products for gatherings. Its catalogue includes Christmas crackers.

The company's owner grins, nearly apologetically at the gag. But the pun has been selected and will feature in future 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 great Christmas cracker pun is not the same as a good joke in itself. It is entirely about the setting - in this instance, the shared amusement of the holiday dinner table with elders, children and possibly neighbours.

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

The Science Of Shared Laughter

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

"Therefore when you are laughing with others at the holiday dinner you are engaging in what's almost certainly a really ancient mammalian social sound," says a neuroscience expert.

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

Scientists have discovered that a absence of these social exchanges can significantly damage mental and physical well-being.

"The people you talk to, and share laughter with, it leads to enhanced levels of endorphin uptake," she continues.

Endorphins are the body'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 particularly awful Christmas cracker gag.

"You're not just laughing at a silly pun with a Christmas cracker," she states. "You are in fact doing a lot of the truly vital task of building, preserving the social bonds you have with the people you love."

What Occurs In the Mind?

But what is truly taking place inside the brain when we listen to a gag?

A tremendous amount occurs in reaction to comedy, it transpires.

Using brain scanning technology, a kind of brain scanner which shows which parts of the brain are more active, researchers have been able to map the areas that get more blood flow.

The research involves scanning the minds of volunteer participants and then exposing them to a database of funny 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 professor.

A joke activates not just the parts of the mind in charge of hearing and interpreting language, but also neural areas associated with both preparation and initiating motion and those involved in vision and memory.

Combine these elements together, and individuals hearing a pun have a complex set of brain responses that underpin the amusement we hear.

The Contagious Power of Laughter

Scientists found that when a humorous phrase is combined with chuckles there is a stronger reaction in the mind than the identical phrase when followed by a non-emotional sound.

"This was in areas of the brain that you would employ to move your expression into a grin or a chuckle," the professor says.

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

Amusement, says the professor, can be contagious.

So what does this imply for the laughter heard at a Christmas gathering?

"People laugh more when you are familiar with people," she says, "and you laugh further when you are fond of them or love them."

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

"It's the laughter. The gag is the dreadful Christmas cracker pun, and it's just a pretext to laugh as a group."

The Quest for the Perfect Cracker Joke

Is it possible to discover the perfect joke?

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

In 2001, a professor established a research project for the planet's funniest joke.

More than 40,000 jokes later, with scores lodged by hundreds of thousands of people globally, he has a clearer idea than many as to what succeeds and what does not.

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

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

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

"The reason is that if no-one laughs – it's the gag's fault, not yours.

"What's interesting about the Christmas cracker puns is that not one person considers them funny.

"It creates a common experience around the gathering and I think it's lovely."