In this talk, I will present a unified account for the discourse particle ba in Mandarin. Previously, people have argued for two bas: ba1 in declaratives, which weakens the assertion; ba2 in questions, which strengthens the question by urging the addressee to answer it (Zhu 1999). I propose that there is only one ba, whose function is to signal that the speaker is exempting herself from a discourse obligation; the effects of ba in different speech acts derive from the different discourse obligations associated with these speech acts. In assertions, the speaker not only expresses her belief that p, but also takes on the obligation to defend p when prompted. ba signals that the speaker exempts herself from this obligation to defend p, and thus weakening the assertion. The discourse obligation tied to a question q? is that all participants need to cooperate to resolve q?. Ba signals that the speaker does not take on this obligation, leading to an apparent strengthening of the question: it is solely the addressee’s obligation to resolve q?.