Would you open an umbrella indoors?

And how do you feel about the number thirteen?

Whether or not you believe in them, you're probably familiar with a few of these superstitions.

So how did it happen that people all over the world knock on wood, or avoid stepping on sidewalk cracks?

Well, although they have no basis in science, many of these weirdly specific beliefs and practices do have equally weird and specific origins.

Because they involve supernatural causes, it's no surprise that many superstitions are based in religion.

For example, the number thirteen was associated with the biblical Last Supper, where Jesus Christ dined with his twelve disciples just before being arrested and crucified.

The resulting idea that having thirteen people at a table was bad luck eventually expanded into thirteen being an unlucky number in general.

Now, this fear of the number thirteen, called triskaidekaphobia, is so common that many buildings around the world skip the thirteenth floor, with the numbers going straight from twelve to fourteen.

Of course, many people consider the story of the Last Supper to be true, but other superstitions come from religious traditions that few people believe in or even remember.

Knocking on wood is thought to come from the folklore of the ancient Indo-Europeans, or possibly people who predated them who believed that trees were home to various spirits.

Touching a tree would invoke the protection or blessing of the spirit within.

And somehow, this tradition survived long after belief in these spirits had faded away.

Many superstitions common today in countries from Russia to Ireland are thought to be remnants of the pagan religions that Christianity replaced.

But not all superstitions are religious.

Some are just based on unfortunate coincidences and associations.

For example, many Italians fear the number 17 because the Roman numeral XVII can be rearranged to form the word vixi, meaning my life had ended.

Similarly, the word for the number four sounds almost identical to the word for death in Cantonese, as well as languages like Japanese and Korean that have borrowed Chinese numerals.

And since the number one also sounds like the word for must, the number fourteen sounds like the phrase must die.

That's a lot of numbers for elevators and international hotels to avoid.

And believe it or not, some superstitions actually make sense, or at least they did until we forgot their original purpose.

For example, theater scenery used to consist of large painted backdrops, raised and lowered by stagehands who would whistle to signal each other.

Absent-minded whistles from other people could cause an accident.

But the taboo against whistling backstage still exists today, long after the stagehands started using radio headsets.

Along the same lines, lighting three cigarettes from the same match really could cause bad luck if you were a soldier in a foxhole where keeping a match lit too long could draw attention from an enemy sniper.

Most smokers no longer have to worry about snipers, but the superstition lives on.

So why do people cling to these bits of forgotten religions, coincidences, and outdated advice?

Aren't they being totally irrational?

Well, yes, but for many people, superstitions are based more on cultural habit than conscious belief.

After all, no one is born knowing to avoid walking under ladders or whistling indoors.

But if you grow up being told by your family to avoid these things, chances are they'll make you uncomfortable, even after you logically understand that nothing bad will happen.

And since doing something like knocking on wood doesn't require much effort, following the superstition is often easier than consciously resisting it.

Besides, superstitions often do seem to work.

Maybe you remember hitting a home run while wearing your lucky socks.

This is just our psychological bias at work.

You're far less likely to remember all the times you struck out while wearing the same socks.

But believing that they work could actually make you play better by giving you the illusion of having greater control over events.

So in situations where that confidence can make a difference, like sports, those crazy superstitions might not be so crazy after all.