Mirror image is a tricky thing, but it's pretty clear when you look at words in a reflection, that mirrors flip things horizontally rather than vertically. Or, at least it seems that that's the case.

For example, when you hold up a sign saying "Food" in the mirror", it reads a backwards "dooF", but the letters are still the right way up. Why is that?

And why, when you raise your right hand, does your mirror-self raise its left hand, but it still moves it up rather than down? How does a mirror know the x direction from the y direction?

This classic 2015 episode of Physics Girl explores what's going on, and it's an answer that's stuck with us.

Because, you see, the reason that things appear to always be flipped horizontally in the mirror actually has nothing to do with the mirror, but with us. It's because we flip them horizontally.

Try holding up that "Food" sign again, but then flip it vertically to face the mirror. You'll then see an upside down, but forward facing, "Food" staring back at you.

The reality is that the mirror isn't flipping things horizontally or vertically at all, it's actually flipping them along the z direction - the one that points out in 3D from a traditional graph.