Verb doubling constructions in Mandarin such as CHI, wo (shi) chi le 'As for eating, I (did) eat', where the first verb is stressed, are perceived as incomplete if not followed by a continuing clause, but only when the copula shi is present do these constructions require a contrastive continuation marked by the coordinator ‘but’. This paper addresses these two puzzles by (i) analyzing verb doubling constructions as contrastive topic constructions where the initial stressed verb is a contrastive topic triggering a set of polar questions, and (ii) taking the copula shi to be a marker associated with contrastive topic. In particular, shi in a verb doubling construction has the presupposition that (a) there is at least one polar question Q which is an alternative to the current question under discussion CQ and entails CQ, and (b) Q has a negative answer. It is shown how the proposal accounts for the puzzles, and how it can be extended to VP doubling and object preposing constructions, both of which demonstrate similar incompleteness effects.