Today was Friday the 13th, a date regarded as unlucky and even supernatural by many Western cultures, especially Americans. Our cultural belief that the number 13 is unlucky is an example of a superstition- a negative term for beliefs that contradict science, sometimes involving magic.
Friday the 13th is just one of many superstitions in Western culture. Other superstitions include believing that breaking a mirror brings 7 years of bad luck, that walking under a ladder foretells a sudden death, and that throwing a pinch of salt over your shoulder brings good luck. Superstition is such a universal part of human culture it is in many of our fictional stories. Have you read Harry Potter? Remember the Grim?
Some of our superstitions are based on logic- walking under a ladder is not good for your health or that of the person on the ladder! But other superstitions have less well known origins. Perhaps the most widely known superstition is knocking on wood—this is done when someone suggests a bad thing might happen, to prevent that bad thing from happening. This particular superstition is thought to come from ancient knocking on wood to call people to prayer- a good way to avoid bad luck was to pray. I think many of these superstitions persist in our culture because they induce positive thinking. We are much more likely to take steps towards a good outcome if we believe we have already avoided a negative outcome.