1. Those Mailboxes That Aren't Really Mailboxes
You know, those grey or green boxes that look like mailboxes and are usually nearish mailboxes, but don't actually accept any mail. What are those? You've wondered for years, but never bothered to look up because you didn't know what they were called and didn't really care that much. Well, according to a bunch of unreliable answers on the Internet, and this somewhat-more-reliable message board from 2003, they're called "relay boxes" and are where letters are stored by one mailman to be picked up by another mailman so the second mailman can deliver them. Or something like that. They're mostly not used anymore because they seem highly inefficient, but the boxes are still there to taunt you when you're trying to find a mailbox so you can pay your rent.
2. St. John's Wort
Why do you even know the name "St John's wort"? How did that get in your brain? Unless you spend a tremendous amount of time perusing herbal remedies at your local drugstore, you probably don't know what this is. You know that it's some kind of medicine-y thing, but what is it? It sounds gross like a fungus or filthy root, but, according to Wikipedia, it's actually a pretty nice flower that is used to treat depression. Some part of this plant (maybe the whole thing? Wikipedia is unclear) is made into pills that you can take if you don't want to take an actual antidepressant that was made by science.If you were also curious about Tipton's weed or goatweed, good news! These are all the same thing. Now you know.
3. What Actually Happened at the End of The Giver
Main Guy and the Baby run away from that place and then the book ends with them hearing music for the first time and seeing Christmas? Remember? What was that? Did they die? They died, right? When you read this in middle school, half your friends thought they'd actually arrived at some lovely Christmas place, and half thought they died. Who was right? At the time, you were confident in your reading, but, recently, when you suddenly remembered The Giver, you weren't so sure. Well, you're not alone. The Internet is full of people who don't know what the hell happened. But, it turns out that there are sequels you never read in which it's proven that they live. Which is actually pretty lame.
Gelatin is made of hooves and horns and stuff. You know and accept that, but how does a cow hoof suddenly have anything to do with Jello? Obviously, chemistry is reallife magic and you can't be expected to understand everything about it, but the gelatin thing bugs you every time you eat pudding. Turns out, it should bug you when you do almost anything because gelatin is everywhere. Peeps, cream cheese, paintballs, film, matchbooks. Gelatin is made from collagen, which is pretty much just natural body glue that holds animals together. We steal that stuff, break its bonds with water, then heat and cool it with whatever cheese/sugar/junk we want to make chewy or jiggly or semi-firm. It's body glue we use to glue candy together. Humans figured out that this could be done. We are gods.
5. That Spot Where Russia, Mongolia, China, and Kazakhstan All Touch
If you're like most people, you spent a considerable amount of your formative years zoning out in class, vaguely staring at giant maps of the world. At some point, you probably noticed that spot where Russia, Mongolia, China, and Kazakhstan all touch. What the hell is that place like? A whole bunch of races and ethnicities happening over there. Who are those people? What do they look like? Is there a place you can stand that allows you to be in all four countries at once? Nope. Turns out that big map was misleading. If you zoom in really close on Google maps, you'll find that all four countries don't actually touch. Huge let down. See?
Still, three countries all touching is pretty decent. Further investigation reveals that there is absolutely nothing going on in those areas though. And the people have exactly that mixed-race look you thought they would, you weird, zoned-out racist.