The origins of the black cat, the broken mirror, and other superstitions.
Bad Luck Superstitions
Walking Under a Ladder
According to early Christian belief, a leaning ladder formed a triangle with the wall and ground. It was considered an affront to the Holy Trinity to walk through a triangle, lest you be considered in league with the devil.
Alternative explanation: Ladders used to be used for getting a body off the scaffold. To walk under it would invite death.
Friday the Thirteenth
In the Bible story of the Last Supper, Jesus says he has chosen 12 disciples, "but one of you is a devil"-a very vivid example of unlucky 13 at table. The next day a Friday, Jesus is crucified. Friday had long been seen as a day of ill omen by the Romans, and was adopted as such by Christianity. Traditionally, Adam and Eve were expelled from the Garden of Eden on a Friday, and Noah's flood started on a Friday. So Friday and 13 make a deadly combination.
Alternative explanation: At the turn of the fourteenth century, the King of France, Philip le Bel, applied for membership in the Knights Templar, a secret order affiliated with the Pope. He was rejected, and out of revenge ordered the arrest of the Templars. Out of 5000, only twenty escaped. The remaining few cursed the day, Friday the 13th, forever.
Saying "God Bless You"
It was once believed that when people sneezed their soul was expelled from their body and could be replaced by an evil spirit. Blessing them brought their soul back.
Alternative explanation: During the Plague in Europe, sneezing indicated that the person was going to die and so was in need of a blessing.
One tradition has it that Judas spilled salt during the Last Supper. Alternative explanation: Salt was used for medicinal purposes in the Middle Ages; disease was believed to be caused by evil spirits. If you spilled salt, throwing it over your left shoulder would hit the spirits in the eye, thus preventing illness.
Avoiding Black Cats
The ancient Egyptians worshipped the Goddess Bast, in the form of a black cat. Christians seeking to eliminate pagan religions later associated black cats with demons.
Break a Mirror
In the Middle Ages, mirrors were very expensive, so breaking a mirror itself was extraordinarily bad luck. It was potentially dangerous because of the shards of glass left over. From a metaphorical perspective, when a mirror breaks, the appearance of reality is fractured in disturbing ways.
Alternative explanation: Ancient Romans believed that reflections in water were actually glimpses of the soul. Disruptions of their reflection affected their soul and brought about bad luck. This was later applied to mirrors.
Opening an Umbrella Indoors
Don't open an umbrella inside the house, or bad luck will "rain" in on you.