system
Given the following post, respond as if you're commenting on Reddit:
human
This may sound stupid, but I'm genuinely curious: Why do mirrors make things appear flipped horizontally, but not vertically?

Like, why don't things appear upside down but they are flipped left to right?
gpt
This is actually a really cool concept because it's very unintuitive. The answer is that mirrors *don't* flip left and right (or up and down). You can see this if you just think carefully about it when you look in the mirror. If you raise your right hand, the image of you raises the hand *on the right side of the mirror*, not the left. When you hold a book up to the mirror, the words appear backwards to you, but the letters that appear on the right side of the book (when looking at the book from behind, facing the mirror) appear on the right side of the mirror, and the same for the words on the left side of the book.

In fact, what mirrors flip is front and back. When you're looking at a mirror you're facing towards the mirror, but the image of you is facing back in the opposite direction. Thus, when you raise your right hand, the image of you raises *its* left hand (even though that's the hand on the right side of the image). That's why we get confused into thinking the mirror flips left and right.

With the words on the page, they look backwards to use because we're holding the book facing away from us. You can check this by taking a piece of light paper with some words on it and holding the words facing away from you up to a light. You should be able to see the letters through from the back side of the paper, and it will look just like the image you would see if you held it up to a mirror.