Hold a book in your hands while standing in front of a mirror. Now turn the book around so you can read it in the mirror. The text is backwards. But how did you rotate the book? Sideways? If you had flipped it over vertically, the text would be upside down instead of backwards. You are the one who reversed the book, not the mirror.
Exactly. Stand in front of a mirror holding a sheet of transparency (such as you would use for an overhead projector) in front of you so that you can read it. Look at the transparency and note that you can read the words. Now look at the transparency in the mirror and note that you can still read the words.
If you rotate the transparency (left/right, up/down, corner/corner, whatever) so that you can no longer read it directly, you'll see that the same rotation has occurred in the mirror.
