A note from Dylan Thurston <dthurston@barnard.edu>: ----- My favorite example is even earlier: in elementary school or middle school, we were learning about the "natural", "counting", and "whole" numbers. (These are varieties of positive/nonnegative integers.) I had a little trouble keeping them straight, and so I asked my father (also a mathematician) which was which. He didn't know and said he used "positive" or "nonnegative"; that made me think something was fishy. I don't think mathematicians are consistent about whether the natural numbers (N) include 0 or not; I suspect this occassionally leads to confusion. Peace, Dylan