Wacky things we get told to believe, for no real reason

Case One

One rainy day during high school, I was sitting inside, eating my lunch when Jeanne, a fellow student, decided to educate me.

She'd obviously noticed that I'd peeled my banana like any normal person would:

But it wasn't good enough for her. She approached me, placed her hands on her hips and declared, "You're supposed to peel it from the other end." By 'other end', she meant this:

"Why?" I asked.
"Because that's how monkeys do it."
I looked around to find my friends nodding in agreement, the way you'd nod in agreement if Gandalf had spoken. Evidently Jeanne was quite wise. But I simply raised my eyebrows. "Are you a monkey?"
"What? No," she replied. I think she was upset, because she took off.

Sure, there could be actual reasons, such as the fact that it requires less effort, to peel a banana from the other end, but "because that's how monkeys do it" isn't, of itself, a reason to do anything. Why would we want to mimic monkeys? Should we speak like monkeys, too? Why not go one step further and build nests instead of houses because, you know, that's how birds do it?


Case Two

I never believed in Santa or the Tooth Fairy, or even the Easter Bunny. I think, during my early childhood, my parents must have said, "They aren't real. Don't listen to anyone," in super stern tones. Not that it would've mattered. Everyone outgrows these beliefs sooner or later.

Strangely, a lot of people never stop believing the myth that the daddy longlegs is the most venomous spider on the planet (and that its fangs are too small to pierce human flesh).

I consider myself quite a gullible person, which is why it surprises me when I think back to that fateful day on which I too was told to believe the myth, and refused. Despite being only eight years old, my undeveloped capacitors for reasoning somehow deciphered that the claim didn't make any sense.

"Its fangs are too short. Well isn't that convenient?"
"Why is an eight-year-old schoolmate telling me this, and not the news, or my dad, or someone who knows stuff?"
"Why are there zero reports of someone with a cut on their foot standing on a daddy longlegs and subsequently dying?"

All of these questions rummaged through my mind, ransacking the furniture and messing up the carpet. But I knew better than to argue with a classmate who placed his Scrabble letters diagonally across the board, and whose facial expression said, "No matter what you say, I know more than you."

Here's some research to debunk the myth forever:

"Supposedly, daddy longlegs possess extremely powerful poison, but their fangs are too short to penetrate human skin. To find out, [Mythbusters'] Jaime and Adam hunted down a host of daddy longlegs and took them to a spider specialist who could milk out their venom. Next, the spider specialist compared the toxicity of daddy longlegs venom to black widow venom. The red-bellied widow won out, busting the myth.
A microscopic measurement of the long-legged spider's fangs proved their miniscule quarter-millimeter length could puncture human skin, taking a double bite out of the daddy longlegs myth."

Case Three

Several years ago, I was having lunch at a café with some mates. I bought a Coke Zero because... I don't know. "I don't want cancer!" I told myself, which is why I don't know. I think I'd never drank Coke Zero before. Anyway, one of these mates (it might have been Jeanne) said, "Eew! You shouldn't drink that."
"Why not?"
"A chemical in the sweetener is used in embalming liquid!"
I think someone changed the subject, because I never got the chance to say, "And water is used in nuclear power plants. Best not drink that, either."