system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is a mirror flipped left to right, but not top to bottom

Why is it when you hold up a newspaper to a mirror, you can't read it due to it, er, being mirrored, but the same thing doesn't happen vertically? I can't wrap my head around this.
gpt
This is because a mirror in fact does not swap left and right. The only dimension that gets swapped is perpendicular to the mirror.

Stand in front of a mirror that lies east-west. Point up, mirror person points up. Point east, west, down, all is the same, Point north, into the mirror, and the image points south, out of the mirror. That is the dimension that is swapped.

When you want to see a newspaper in a mirror, what way did you turn it? Left for right? In that case, it is you that turned it, not the mirror. You could have turned it top-for-bottom, and the the writing would then have been upside down.

Why do people say that a mirror swaps left and right? Simple - they say 'what movement would mirror person have to have made to be where they are?'. The answer your brain comes up with is that they would have turned around, as people normally do not do handstands when they want to walk the other way. So it applies that transformation, a simple rotation, and then starts to notice where that transformation is wrong - as in, a rotated person would have swapped their left hand for their right, but the mirror did not do that.

Some people's brains happen to apply the right transformation to objects in the mirror - sort of 'inside out', or 'back for front'. They have no problems reading items in the mirror, and are more likely to think 'What on earth is an "ecnalubma"' when they see one in the rearview mirror.