Why do mirrors flip images horizontally but not vertically?

Mirrors flip images horizontally but not vertically because they reflect things straight back, like a perfect copy, but from side to side.

Imagine you're standing in front of a mirror and raising your right hand. The mirror shows you raising your left hand, it looks like the mirror flipped your image left to right, or horizontally. But if you jump up and down, the mirror still shows you jumping up and down, not flipped vertically.

Think of a mirror like a door that opens backward. If you write on a piece of paper and hold it up to the mirror, it looks like someone wrote backwards, but only from left to right. That’s why we say mirrors flip things horizontally, not vertically.

Mirrors don’t actually know what’s “left” or “right.” They just reflect everything they see, and since you're facing the mirror, your left side is reflected on its right side. So it looks flipped, but only from your point of view!