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

This one-liner is met by moans that resonate through a warehouse in London.

We're at a humor-evaluation session with a firm that produces supplies for social events. Its catalogue features Christmas crackers.

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

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

The secret to a great holiday cracker pun is not the identical as a good gag in itself. It is entirely about the context - in this case, the shared amusement of the Christmas dinner table with grandparents, children and potentially friends.

"You want the joke to be a thing that unites the child in harmony with the 80-year-old," she adds.

The Science Behind Communal Amusement

Gathering to experience communal laughter is not only ancient, scientists say, it is likely to be pre-human.

"Therefore when you are chuckling with people at the Christmas table you are engaging in what's almost certainly a truly primordial mammalian social sound," explains a professor.

Shared amusement, she explains, helps make and maintain social bonds between people.

Researchers have discovered that a absence of such interactions can significantly harm mental and physical health.

"The people you talk to, and laugh with, it leads to increased amounts 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 reaction to enjoyable experiences, such as laughing with friends over a particularly terrible festive cracker gag.

"It's not simply chuckling at a foolish pun with a Christmas cracker," she says. "You are actually doing a lot of the really vital work of building, preserving the social bonds you have with those you care about."

What Occurs In the Brain?

But what is truly happening within the mind when we listen to a gag?

An awful lot happens in response to comedy, it transpires.

Employing functional magnetic resonance imaging (fMRI), a type of brain scanner which shows which areas of the brain are working harder, scientists have been able to map the areas that receive more blood flow.

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

"During the study we got a very interesting pattern of activation," notes the neuroscientist.

A gag activates not just the parts of the brain in charge of hearing and interpreting language, but also brain areas involved in both preparation and initiating motion and those involved in sight and memory.

Combine these elements together, and people hearing a pun have a sophisticated set of neural reactions that support the amusement we hear.

The Infectious Nature of Laughter

Researchers found that when a humorous word is paired with laughter there is a greater response in the mind than the same phrase when accompanied by a neutral sound.

"This was in areas of the brain that you would employ to contort your expression into a smile or a chuckle," she explains.

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

Amusement, according to the expert, can be contagious.

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

"You laugh harder when you are familiar with others," she says, "and you laugh further when you are fond of 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 gag itself, but from the response to it.

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

The Quest for the Perfect Cracker Joke

Is it possible to find the ultimate gag?

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

In 2001, a professor established a scientific project for the planet's most humorous joke.

More than 40,000 jokes submitted, with scores provided by hundreds of thousands of participants around the world, he has a better understanding than many as to what works and what fails.

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

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

The more "awful" the gag, he says the better.

"This is because if nobody finds it funny – 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."