I remember being about 9 (??) And doing a homework sheet or quiz or something with a drawing of an empty glass and being asked to draw a line so it would look half full. So I obviously drew a horizontal line through it. And then a drawing of an empty glass on its side and ditto with making it look half full. I remember so clearly thinking these were so ridiculously easy they must be trick questions and worrying I'd missed the point, and then being shocked that some of my friends drew a vertical line instead of horizontal in the sideways glass...