Why does Friday the 13th, and not, say, Tuesday the 13th (which we all experienced uneventfully last month), fill us with terror and a sense of impending doom? Despite the definition of "superstition" as a belief or practice resulting from ignorance, fear of the unknown, trust in magic or chance, or a false conception of causation (thank you Merriam-Webster) it's possible that at least a few of our superstitions are in fact meaningful, even practical or helpful. In honor of Friday the 13th, we've gathered the "truths" from around the Internet (and our own personal experience) behind the various superstitions that cause us to avoid ladders and black cats; eat apples daily; and hang on to those grimy old bunny paws just in case. Maybe, just maybe, there is rhyme beneath some of our reason. Or maybe we're nuts. 

Black cats. It was believed that witches and demons could take the form of a black cat. More practically, however, do you really want to cross any cat's path? Making eye contact is so awkward. And then there's that thing about cats making you, actually, insane. 

Walking under a ladder. Apparently, the open ladder form, or a triangle, is a symbol of life or possibly the Holy Trinity, something one does not want to tempt by walking through. However, more practically, people drop things from ladders, and an unsecured ladder in high winds can be dangerous indeed! Best to avoid at all costs. Pro tip: "If you do accidentally walk under a ladder, you can counter the bad luck by placing your thumb between your index and middle finger." And step lively.

Throwing salt over your shoulder. Say the devil is standing behind you at all times (as the superstition goes). How are you going to get rid of him? Grab the table salt that's handy and toss it backwards, over your shoulder, aiming for his eye. That'll show that jerk. However, now that we understand the risks of high-sodium diets, this mechanism to evade bad luck is something of a double whammy. Get rid of the devil and ensure good cardiovascular health, too!

Knocking on wood. Legend had it, good spirits lived in trees. Knock on something and wake him up! Or don't, because of splinters. And how do we know good spirits are morning people? 

Umbrellas in the house. Some common theories about this: Opening an umbrella in the house insults the sun gods. Or it offends the rain gods, who are like, why are you opening your umbrella inside? Or it bothers your mom, who thinks you're going to break something, because—oops. 

Breaking mirrors. See above, your mom. Also: Glass shards. Also, the reflection in the mirror is supposed to be your soul, and breaking it is not good for you. But mostly it's a lot of cleanup. 

A rabbit's foot brings good luck. This may be the grossest commonly held superstition. Grosser: One theory is that the luck of a rabbit's foot was thought to be so because it's actually the foot of a shapeshifted witch. Some said for the greatest luck you should capture this rabbit and take its foot on Friday the 13th. Others said you were just lucky because you got to eat the rabbit. PETA, we assume, does not agree with any of this.

An apple a day keeps the doctor away. This is just healthy, and may have originated with a proverb. Just wash it first. 

Saying bless you when someone sneezes. This goes back to the time of the plague, when people who started sneezing violently were expected not to make it. "The soul was thought to leave the body through the nose upon death, so a powerful sneeze was thus considered an ominous event." Today of course you say bless you, or the less religiously assumptive gesundheit, so as to avoid being thought a crass, mannerless heathen—or maybe just because the office is way too quiet.

Picking up a penny for good luck. Because you could buy something with it, essentially. (Pennies were worth more back then.) Now, with inflation, there's no reason to pick it up, except for exercise purposes. 

Crossing your fingers. This is the sign of the cross, and was said to keep evil spirits from messing with your good luck. On the bad side, it may result in uncomfortable, unusable fingers. 

Friday the 13th. It's thought that there are Christian and Norse roots to this one. On the Norse side, it was believed that number 13 was unlucky due to a 13th evil demigod joining the mythological 12. As for the Christian beliefs, it's said that Christ was crucified on a Friday, and Judas, who betrayed him, was the 13 guest at the Last Supper. (It was also thought that having 13 people at a dinner party would result in the death of one of the diners.) But more practically: Plates don't come in sets of 13. 

Image via Shutterstock by Apple's Eyes Studio.