So I'm reading "Flatland" by Edwin Abbot to my kids, and one of 'em suddenly says, "Flatland is a vertical plane." I stared at him for a moment, and the scales fell from my eyes, and despite having read the book many times over many years, I realized he was right about something I'd never seen before. The clues were all there - the rain falls from the 'north' and the flatlanders all feel a vague pull to the 'south', coresponding to our up and down. I'd never even bothered to think about this, I just assumed it was horizontal like a table. I just ran it by R2 (Who's kind of a math guy) and he hadn't realized it until just now, either!
Way to go, kid!