That reminds me of an episode of Barney Miller:
Jack Soo’s character is afraid of dying (I think). Someone says, ‘It’s just like going to sleep.’ He replies, ‘I’m an insomniac.’
Why people who don’t understand something don’t search for it, or make a thread about it. 
Also, why when one defender of the “Big Lie” tactic of politics has been defeated in an argument, why another steps in with the exact same logic, and acts like the rest of the thread didn’t exist.
Infinity of big numbers and space.
and how women think sometimes. sigh
People who deliberately cause physical pain to kids
Imaginary numbers. The square root of -1? Couldn’t possibly be!
I can’t get why other people don’t agree with me on certain topics. I mean, it’s so very clear and simple. I totally get that if they’re lacking a certain piece of information, they may have drawn an incorrect conclusion, but why don’t they revise their views once I explain the way things really are?*
*The above was written with tongue firmly in cheek. Mostly.
You really have to try it sometime. It’s maybe the thing I miss most about living in Oregon. Nothing like waking up, making eggs, pancakes, bacon and a pot of coffee and knowing you got nothing to do for the next few hours except digest and watch foots.
Not to mention getting to bed at a reasonable hour on Sunday and Monday night.
Money.
I still don’t understand where “new” dollars come from.
Right now, there are trillions of US dollars in the world.
200+ years ago, where were none.
Somehow it’s involved with loans, but I still don’t get how the money supply grows.
They make it up as they go along. 
Almost exactly what I was going to post.
I don’t understand how when I explain how things are to people, they insist on sticking to their ignorance. Why would someone want to be ignorant. I mean, if my position was wrong, I wouldn’t have it. It’s just common sense.
Compound interest
See, I have the same issue. For the life of me I can’t understand what’s going on in Trading Places and I think I’ve seen that movie 349803840 times. So when I read Vtwin’s very well written explanation of the market, here is what I see:
“think of the stock market as a big game of hot potato… blah blah blah dividends. blah blah blah companies equity. blah blah blah logistical termination” and I just completely tune out. Why were Billy Ray and Winthorp on the floor? What did they have to sell? What were those little slips of paper? Why could they just walk on the floor and start yelling stuff and everyone came running over?
I just don’t get it. Falls to the floor sobbing.
My brother-in-law doesn’t read books. Evidently there are other people just like him. How is that possible?!
Why a cow-orker thinks it’s okay to deliberately run over animals because she doesn’t like cats and dogs.
There is no such thing as nothing. Because it has to be something in order to be nothing. And where does it start? What came before eternity?
Special relativity. How the hell can light travel at the same speed relative to something else, no matter how fast that something else is going?
I get it, but I don’t think I’ll ever get it.
And another for people who don’t ever read.
I’m standing on the Earth. Beneath my feet is seven-thousand-and-some miles of rock and metal, and then it stops. Above my head is fifty miles of air, another fifty miles of barely measurable wisps, and then… nothing. Absolutely nothing. For a really, really, really, really, really long way. The nothing above my head is only a third of the distance from me than my uncle is if I drive straight south. And it’s really, really, really, really, really, really, really big.
I think about this a lot, but when I try to get my head around it, I fall back into mental metaphors and half-remembered diagrams and stuff.
Oh, and that big bright round thing in the sky during the day? We call it the Sun, but really it’s a star. Whoa.
I completely agree. I’ve lived on this coast for over 3 years now, and I still look at the schedule and think, “Ok, so that’s the 1pm game, which is really 10am, and that’s the 4pm game, which is really 1pm.” Also, the kick-off of the first game of the day is the time to crack open a beer, because you got up at 8 or 9, mowed the lawn or something, had some lunch, and now it’s time to relax and watch some football. I have nothing against having a beer at 10am, but it’s probably not something I should do on a weekly basis.
Tires.
Specifically, the fact that while hurtling down the road, any single point on your tire becomes motionless for an instant while in contact with the asphalt. Then it accelerates up and around, down, and finally comes to a complete halt again. Over and over - for however many miles you travel.
This bugs me for some reason.
Trains. On every train, even the fastest bullet trains in Japan, there is always a part of the wheel going backwards.
Accents, you know like southern, northern, southern californian…etc. I mean I can’t fathom people in the SAME country talking so different. And why, when you speak another language does it sound different than a native speaking it? I have always wondered too, what does an American accent sound like to others? How did we all come to sound so dofferent?
You aren’t the only one…I was talking to a Korean today who went to Florida and was amazed that Florida english and Wisconsin english were so different.
I can wrap my head around distances. If it is farther than a few miles, I measure in hours. And a few miles is anything longer than walking distance and too far to bike. Anything within walking distance is ‘near’ or ‘right by’
How electricity works. There is voltage and resistance and current and they all are interrelated and can create a magnetic field.