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

This quip is greeted with moans that echo through a warehouse in the capital.

This describes a joke-testing session with a firm that produces supplies for gatherings. Its repertoire includes Christmas crackers.

The firm's owner smiles, nearly apologetically at the joke. But the joke has been selected and will feature in upcoming crackers.

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

The secret to a great Christmas cracker pun is not the same as a good gag in itself. It is all about the context - in this case, the shared amusement of the Christmas meal with elders, kids and possibly friends.

"You want the joke to be something that brings the eight-year-old together with the 80-year-old," she adds.

The Science Behind Shared Amusement

Gathering to experience communal laughter is not only nothing new, scientists say, it is likely to be pre-human.

"Therefore when you are chuckling with people at the holiday table you are engaging in what's very likely a really primordial mammalian social sound," says a neuroscience expert.

Communal laughter, she explains, aids in forge and strengthen social connections between individuals.

Researchers have discovered that a lack of such social exchanges can significantly harm both psychological and bodily well-being.

"Those you talk to, and laugh with, it results in increased levels of 'happy chemical' release," the professor adds.

These natural chemicals are the body's "happy chemicals" and are released both to reduce tension and discomfort and in reaction to pleasurable activities, such as chuckling with friends over a particularly terrible festive cracker gag.

"You're not just laughing at a foolish pun with a Christmas cracker," she states. "You are actually doing a lot of the really important work of making, maintaining the connections you have with those you love."

Which Occurs Inside the Brain?

But what is actually happening inside the brain when we hear a gag?

A tremendous amount happens in reaction to comedy, it turns out.

Using brain scanning technology, a type of neural imager which indicates which areas of the brain are working harder, scientists have been able to chart the regions that get more blood.

Testing entails scanning the brains of healthy subjects and then subjecting them to a database of funny phrases, paired with either a neutral sound, or recorded chuckles.

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

A gag stimulates not just the parts of the mind responsible for auditory processing and understanding language, but also neural areas involved in both preparation and starting movement and those linked to sight and recall.

Put these elements together, and individuals listening to a joke have a sophisticated series of neural responses that underpin the laughter we hear.

The Contagious Nature of Laughter

Scientists found that when a humorous word is paired with chuckles there is a greater reaction in the brain than the same word when followed by a neutral sound.

"This was in areas of the mind that you would employ to contort your expression into a grin or a chuckle," the professor explains.

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

Amusement, says the expert, can be infectious.

So what does this mean for the laughter heard at a Christmas table?

"People laugh harder when you know people," she says, "and you laugh further when you are fond of them or care for them."

When it comes to festive cracker jokes, she says, the feel-good factor is more probable to be triggered not by the joke in itself, but from the reaction to it.

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

The Search for the Ideal Festive Pun

Is it possible to discover the perfect gag?

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

Years ago, a professor established a scientific project for the world's funniest gag.

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

The ideal festive cracker pun must be short, he says.

"But they also need to be bad gags, jokes that cause us to moan," he adds.

The increasingly "awful" the gag, he states the better.

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

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

"That's a common moment at the gathering and I believe it's wonderful."