We tend to think that mirrors reverse left to right, but in fact they reverse “back to front,” along an axis perpendicular to the mirror’s surface. Our confusion arises when we misinterpret this.

“[I]magine a back-front reversal of yourself, with your nose, face, eyes, and so forth pushed through to the back of the head, and your back somehow oozed through to the front,” writes University of Auckland psychologist Michael C. Corballis. “You might then ‘feel’ your watch as having remained on the left wrist (say), while back and front have reversed. However it is likely that you will also experience a strong compulsion to recalibrate your internal axes, and then feel the watch to be on the right wrist. In short, a back-front reversal is reinterpreted as a left-right reversal.”

(Michael C. Corballis, “Much Ado About Mirrors,” Psychonomic Bulletin & Review 7:1 [2000], 163-169. More mirror puzzles.)