Watch where you're walking
Don't walk under a ladder. This well-known superstition has several possible origins, one of which dates back to medieval times when a leaning ladder was thought to resemble the gallows. By walking under one, you were playing out your own execution. Plus, it's a bit dangerous.
Black cats crossing your path
As companion animals for humans for thousands of years, cats play all sorts of mythological roles. In ancient Egypt, cats were revered; today, Americans collectively keep more than 81 million cats as pets.
So why keep a black cat out of your path? Most likely, this superstition arises from old beliefs in witches and their animal familiars, which were often said to take the form of domestic animals like cats.
Careful with that mirror
According to folklore, breaking a mirror is a surefire way to doom yourself to seven years of bad luck. The superstition seems to arise from the belief that mirrors don't just reflect your image; they hold bits of your soul. That belief led people in the old days of the American South to cover mirrors in a house when someone died, lest their soul be trapped inside.
Like the number three, the number seven is often associated with luck. Seven years is a long time to be unlucky, which may be why people have come up with counter-measures to free themselves after breaking a mirror. These include touching a piece of the broken mirror to a tombstone or grinding the mirror shards into powder.
Knock on wood
This phrase is almost like a verbal talisman, designed to ward off bad luck after tempting fate: "Breaking that mirror didn't bring me any trouble, knock on wood."
The fixation on wood may come from old myths about good spirits in trees or from an association with the Christian cross. Similar phrases abound in multiple languages, suggesting that the desire not to upset a spiteful universe is very common.
No umbrellas inside
… And not just because you'll poke someone's eye out. Opening an umbrella indoors is supposed to bring bad luck, though the origins of this belief are murky. Legends abound, from a story of an ancient Roman woman who happened to have opened her umbrella moments before her house collapsed, to the tale of a British prince who accepted two umbrellas from a visiting king and died within months. Like the "don't walk under a ladder" superstition, this seems to be a case of a myth arising to keep people from doing something that is slightly dangerous in the first place.