A lot of people give the correct (but nonsequitur) answer "mirrors don't flip text horizontally or vertically, the flip it through the plane of the mirror". But it is still the case that if you hold a book up to the mirror almost certainly you'll be reading right-to-left, but *still* top-to-bottom! That needs an explanation! And the answer is this: you broke symmetry when you decided to hold the book up to the mirror – you probably rotated the book about the vertical axis, not its horizontal axis. I first heard this from @shachaf. If you have a book in your hand and you simply tilt it forward then you'll be reading the text left-to-right bottom-to-top in the mirror, but that's a much less natural motion, and is not usually how you present text to the mirror. It's crazy to me how often folks get this one wrong. Like, they hear "mirrors actually mirror through the plane of the mirror" and nod sagely, and forget to explain the original phenomenon that almost all text I see in mirrors requires me to read right-to-left+top-to-bottom.