Why Mirrors Reverse Left and Right but Not Up and Down

Stand in front of a mirror and raise your right hand, and your reflection appears to raise its left hand -- yet the reflection never seems to stand on its head or swap its feet for its scalp. This asymmetry confuses people because a mirror doesn't actually reverse left and right at all. What a flat mirror actually does is reverse front and back: the direction pointing toward the mirror gets flipped to point away from it, while up-down and left-right are both left completely untouched. The illusion of a left-right swap comes entirely from how a person interprets the reflection, not from anything the mirror itself is doing. When you imagine your reflection as a person facing you, your brain assumes that person turned around to face you, the same way a real person would turn to greet you. But turning around to face someone reverses left and right relative to you, since what was your reflection's left hand is now on your right side once you mentally rotate them around a vertical axis. If the same reflection were instead imagined as having flipped forward, tipping over to face you like a falling tree, the illusion would reverse top and bottom instead of left and right. The mirror treats every direction the exact same way; it's only the specific mental rotation a viewer automatically applies to make sense of a facing reflection that decides which pair of directions appears swapped.