The Mandarin sentence final particle *ba* has been described as expressing un- certainty or soliciting agreement in declaratives (‘weakening’ an assertion, Han 1995, Li 2006 a.o.). However, unlike English ‘weakeners’ like final rises or epistemic modals, *ba* in interrogatives appears to ‘strengthen’ a question to a demand: “you must answer this question”. This is sometimes taken to show how discourse particles can modify the default discourse effects of declaratives and interrogatives (Ettinger and Malamud, 2014). Others (Chao 1968 Zhu 1999) postulate a lexical ambiguity: *ba1* in declaratives, which weakens an assertion; *ba2* in interrogatives, which strengthens a question. I discuss new data showing that *ba*-declaratives are not always ‘weak’ and *ba*-interrogatives are not stronger than unmarked interrogatives in the same context, which is not explained by current theories. I propose that, uniformly, *ba* is used to mark its prejacent as contributing to an issue it represents as open, and the correlation with clause types is an epiphenomenon.