Why Do Mirrors Flip Left and Right But Not Up and Down?

When you look in a mirror, it seems like your left hand becomes right. But the mirror isn't actually turning you sideways! It is more like it is flipping you forward and backward, like if someone pushed you through a window.

Imagine you are wearing a shirt with writing on it. If you turn around to face a friend, the writing looks backwards because your chest moved toward them. A mirror does this with depth. It swaps what is in front of you with what is behind you.

Our faces have two eyes side by side but only one nose up top. We are used to turning things horizontally because we walk that way. When the mirror flips us forward, our left ear stays on the left but appears on the right from our perspective. It feels like a side swap.

We also look straight ahead at mirrors. If you lie down in front of one, your head is still up and feet down. The mirror does not flip you upside down because gravity keeps us oriented vertically. We trust our eyes more than the physics!