What are your favourite mathematical "quips"?

I don't really mean math jokes, more the little witticisms we've all picked up over time. My two favourites are: "Proof by intimidation", which I first heard from one of my professors after an especially bewildering set of arguments from an outside speaker at a seminar "Mathematics is locally trivial", which I first heard from my functional analysis professor and have always found a little comforting ever since Puns also welcome of course :)