I encountered this old puzzling question on xkcd: "When you look at things on a mirror, how come they're reversed left to right and not top to bottom? What's so special about the horizontal axis?" and decided to get to the bottom of it once and for all. So this is how it goes:
Mirrors actually show things as they are: left is left, right is right, top is top, bottom is bottom. The reversal actually occurs when you turn the object around to look at it in a mirror. Say you were holding towards you a cardboard with the word "CARDBOARD" written on it. Now if you wanted to look at it in a mirror, you'd have to turn it around, and that's the part where it gets reversed. People usually turn it around horizontally, so left becomes right and right becomes left. If the cardboard were transparent, then you would be able to see that you just turned the letters around horizontally with respect to yourself. So, if instead, you flipped the cardboard around top-to-bottom (try it), you would see in the mirror that the letters were reversed top-to-bottom, and NOT left-to-right. Now you might say "but that's because I made them upside-down!", but that's the point. Even in the "normal" case, you made the word right-to-left.
So what about words written on your t-shirt? You don't reverse your t-shirt around horizontally to see it. Well actually you do, when you wear it, as opposed to when you are holding it out in front of you. If you somehow wore your t-shirt by flipping it vertically and sliding the neck-hole down to your waist, then you would see the letters upside-down in the mirror.
Also, if you look at a word on a wall, and then turn around HORIZONTALLY to look at it in a mirror, then it's reversed left-to-right. On the other hand, if you turned around vertically by, say, doing a hand-stand, then you would see it reversed upside-down. Sheesh, all the ways this horizontal turning-around has crept into our lives! I guess it's because we're conditioned (by evolution and gravity, perhaps?) to regard objects turned around horizontally as the same as the original object (i.e. a tiger turned away from you is the same as a tiger turned towards you, albeit one may be a bit less dangerous). An object turned over on its head, on the other hand, seems to us as a bit odd.
...Then there's the question: what if you turned around diagonally? Can you turn around diagonally? Well, at least you can turn around a piece of cardboard diagonally, in which case, left would be up, and... gah, that's a question for another time!
