grab a piece of paper with text on it. place it in your hands as if you were about to read it. Now flip it so a mirror in front of you can see it. The text is

grab a piece of paper with text on it. place it in your hands as if you were about to read it. Now flip it so a mirror in front of you can see it. The text is right to left, isn't it? Now notice that, in order to "show it to the mirror", you applied an horizontal rotation to the paper. Try placing the piece of paper as it was at the beginning, so it's readable to you again, and then "show it" again to the mirror, but this time flipping it with a vertical flip. There you go, the text is left to right but upside down. Vertical flip! You're probably thinking "yeah but the original is upside down too" to which I would reply, yes, and in the first case the original was also right to left from your position - you just coudn't see it because the paper is opaque. writing in a piece of glass , you can see it left to right in the mirror when it's left to right in real life.