The mirror doesn't do the swapping - you do. If you hold a piece of paper so you can read it, you have to turn it around to show it to a mirror. Did you turn

The mirror doesn't do the swapping - you do. If you hold a piece of paper so you can read it, you have to turn it around to show it to a mirror. Did you turn it up/down? The mirror shows it upside down. Did you turn it left/right? The mirror shows it reversed left/right. When we interact with other people, they are normally rotated horizontally from us, not vertically, so rotating a paper horizontally is appropriate to give them the same view of the object we have. If we typically met face to face with a vertical rotation - one of us standing on his head, but right hands both on the same side, we would think a vertical rotation was the appropriate thing to do to show a paper to someone else, and would wonder why mirrors showed things upside down.