Why do mirrors reverse left and right but not up and down?

The answer

Mirrors don't reverse left and right - they reverse front and back. When you face a mirror, your reflection faces the opposite direction along the axis perpendicular to the mirror's surface. We interpret this as a left-right reversal because we mentally imagine the reflection as another person who has turned around to face us. If someone actually turned around, their left and right would swap. The mirror doesn't flip anything laterally; our brain's mental rotation creates the illusion. Hold text up to a mirror and you'll see it's reversed because the front-back axis is what truly flips.