Why Do Mirrors Reverse Left and Right But Not...

So Blake keeps asking me (why he asks me I don't know, maybe he thinks I need practice looking stuff up) why mirrors reverse left and right, but not up and down. Well, uuuh, I dunno, but I thought I would look it up for him since he hasn't bothered to do so himself. (It's called enabling.)

I found this page which is notable not so much for its comprehensibility (I actually couldn't figure out what the hell they were talking about, not that I tried very hard) but for its patient explanations of basic social rules. "It's not nice to treat people like they're stupid by deliberately misinterpreting their questions, even if it is lots of fun."

A better explanation is this socratic treatment, if you're interested.