Once upon a time, when I was a teaching assistant, teaching a class of the kind mockingly called "Math for Poets," an obnoxious freshman said to me, "Zero isn't a number." I have forgotten my answer, but I remember finding her remark a shocking expression of profound ignorance. Years later, it dawned on me she was right! If I say "I own a number of calculus books" or "I have a number of friends at the Courant Institute," I don't mean zero books or zero friends. I don't even mean one book or one friend. I mean two or more. That's what "number" means in plain English. I read recently that the famous phenomenologist Edmund Husserl meant by "number" something greater or equal to 2. So did Plato. In mathematical talk, "number"has several meanings. None is the plain English meaning. The ordinary math teacher, like me back then, is so deeply embedded in math lingo that he/she doesn't notice the inconsistency. But the inconsistency can confuse students. I say "math lingo,"not language. It's a jargon, a semidialect of English (or some other natural language), not a complete language. You can't say " I have a headache" or "You bore me" in math lingo. In math lingo, a straight line is the simplest example of a curve. In plain English, quite otherwise: a straight line isn't a curve, and a curve isn't a straight line. In English, what we call a " line segment" is just a " line." What we call a " line" is "an infinite line." "Difference," "product," "factor," "prime" all have different meanings in plain English and in math lingo. I may ask a student, "If you subtract zero from zero, what's the difference?" While answering math-linguistically, "zero," she may be thinking, plain-Englishly, "That's right! Who cares? What's the difference?" In English, "adding" increases what you've got. In math lingo, it may increase it, decrease it or neither, depending on whether you happen to be adding something positive, negative or zero. Correspondingly, subtracting decreases. In math lingo, it may decrease or increase or neither. In English, "adding" and "subtracting" are opposite. In math lingo, they're opposite, and yet they're the same! For adding a number is the same as subtracting some other number (its negative). In English, "multiplying" means repeated adding. It makes things bigger. In math lingo, multiplying makes them bigger, smaller, or neither, depending on what you multiply with. Correspondingly, "divide" means cut into pieces, possibly equal pieces. In math lingo, "divide" is the same as "multiply," in the sense that dividing by a number other than zero is the same as multiplying by some other number (its reciprocal).