The “mirror paradox” (why does a mirror reverse left and right but not up and down?) has been around for quite a while. I think I first encountered it reading a Straight Dope book back in the 1970’s. My advisor asked several of us the question when I was in graduate school in the ‘80’s. If you Google “mirror paradox” you will get a fair number of hits (like this, this and this) that explain the paradox. Most of the explanations are long and involved, far longer and more involved than they need to be.
I should note that I enclose mirror paradox in quotes because it’s not really a paradox. It’s another case in which the problem is stated in misleading terms.
First, here’s the answer: A mirror does not reverse left and right, or up and down; it reverses in and out. To prove that to yourself, take and arrow or anything else with a pointed end, and stand in front of a mirror. Point the arrow to your left. The mirror image of the arrow also points to your left. Point it up, and the reflection also points up. Point the arrow in towards the mirror and the reflected arrow points out, towards you.
I sent this explanation in to the Straight Dope people, and they responded that I was right (or course) but that the explanation wasn’t satisfying. I think they wanted more philosophy.
Looking at left and right hands is confusing, because you tend to assign leftness or rightness based on the assumed perspective of an individual. In the case of the mirror paradox, you assign leftness or rightness to your reflection based on what an actual person would perceive if the mirror image were an actual person. But just ignore the imagined person in the mirror that seems to be raising the wrong hand. It’s not really a person, it’s just a reflection. Use an arrow to avoid confusion.
And now the last of my trick questions. This is not really a trick question, but it is kind of a nerdy question. Everyone knows that air is light and water is heavy. In fact, water is more than 800 times heavier than air. Most people have heard a weather forecaster talk about relative humidity, and understand that “humid” air contains water vapor, or water molecules, mixed in with the air. The question is this: is humid air heavier, the same weight, or lighter than the same volume of dry air?