Another way to put it is that superstition is an irrational or nonscientific belief in the existence of certain powers operant in the world, with positive or ill (usually ill) effects, and therefore a concomitant belief in the counter-effects of amulets, tokens and such, and the power of certain actions (or avoidance of some actions, such as not walking under a ladder) to diminish or deflect these ill effects and/or to promote the positive influence (i.e. crossing one's fingers, or rubbing a beneficent stone, for good luck) of these indeterminate and usually unnamed powers.
A few other familiar examples:
- Breaking a mirror will result in years of bad luck.
- The number 13 is "unlucky" and the numbers 7 and 9 are "lucky."
- It will bring you bad luck if you step on the cracks in a sidewalk.
- Finding a four-leaved clover will be a boon to your fortune.