This might save you a click: The mirror doesn't flip anything. Your image in the mirror faces back at you, and to match that image, you need to turn around 180 degrees. Since there's gravity, and we're standing on the ground, we do that by turning left or right, making it seem like the mirror had flipped the image horizontally. But if we were in space, we could instead turn around by doing half a somersault, making it seem like the mirror had flipped the image vertically. What makes things a bit confusing is that the left and right sides of a human are mirror images of each other, so a horizontal flip seems like it makes less difference than a vertical flip.