Why a Mirror Flips Left-Right but Not Up-Down

A bathroom mirror doing the flip everyone complains about, set next to a floor mirror and a still lake doing a different flip that nobody has ever called strange, so you can watch, side by side, that the glass isn’t the one making the choice.

Before you start. All you need is a mirror, or a clear memory of standing in front of one, and five minutes to test what you think you know instead of trusting the answer that pops into your head. Nothing below needs geometry, physics, or a word longer than the ones you already use.

The idea in 30 seconds

A flat mirror only ever reverses depth, the direction running straight through the glass, and nothing else.
The “left-right swap” isn’t the mirror’s doing. It’s the U-turn you imagine performing to face your reflection like a person, and that imagined turn is what swaps the sides.
Lie on your side, or look into a lake, and the very same flip gets reported as “top-to-bottom” or “up-down” instead, proof the swap was always yours to name, not the glass’s to cause.
Objects that keep their shape but come in a left and a right version no rotation can merge are called handed: gloves, screw threads, and some molecules, whose two hands can smell like two different things entirely.
One force in physics, the one behind a certain radioactive decay, breaks the assumption that nature doesn’t care about mirrors at all, which is how, since 1957, “which way is left” finally has an answer you could radio to a stranger.

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