Pencil lead. While modern pencils don’t contain lead, that’s still what we call the graphite compound. When real lead was used in pencils it was mined and nearly everyone has used a pencil at some point in their life.
Alfred is at the bank to cash his $200 check. He tells the cashier he would like some one dollar bills, ten times as many two dollar bills and the rest in fives.
How many of each denomination does the cashier need to give Alfred?
We know that in order to give the rest of the amount in fives, the sum of the one and two dollar bills needs to be divisible by five (i.e. end in 0 or 5).
If we start with a single one dollar bill, we’d need ten two dollar bills to satisfy the request, making $21. But we need a sum that is divisible by 5. So we keep going up, like so:
The answer is the word “wrong”. When it’s pronounced “wrong” (rhyming with song) that’s the correct pronunciation. However, if you were to pronounce the word as “right” (rhyming with night) that would be an incorrect pronunciation and be wrong.
Most of you have two eyes. I only have one. Most of you have eyeballs. I do not. Your eyes aren’t dangerous, neither are mine. But all together, I am extremely dangerous. You can see things with your eyes. I can’t see anything, even though the air is clear where my eye is.
A variation of this brain teaser includes laser printers (they were invented by Gary Starkweather) and bulletproof vests (invented by Casimir Zeglen using silk, though modern vests use Kevlar, so it’s halfway right).