Requires no previous knowledge of the subject matter and is less than ten lines long. Trust me, It's true. Combinatorists do it as many ways as they can. Combinatorists do it discretely. Algebraists do it in groups.
Algebraists do it in a ring.
Algebraists do it in a field. Analysts do it continuously. Real analysts do it almost everywhere. Pure mathematicians do it rigorously. Topologists do it openly.