Linguists seek to understand the semantics of expressions in human languages. Certainly there are many natural language expressions--operators in the wild, so to speak--that control evaluation in ways that are familiar from programming languages: just think of the natural-language counterparts of if, unless, while, etc. But in general, how well-behaved are control operators found in the wild? Can we always understand them in terms of familiar programming constructs, or do they go significantly beyond the expressive power of programming languages?