In English, the answer to a yes-no question can be a bare answer particle, yes or no. In Chinese, represented in the paper by Taiwanese (Southern Min), the answer can be a bare affirmative answer particle si a, but the negative answer particle m-si must be supplemented by a spelled out clause, possibly reduced by VP-ellipsis. This is explained under the following postulates: The answer to a yes-no question is always a full clause. The answer particle is merged in focus position in the C-domain. In English neutral yes-no questions the IP is headed by a polarity feature with open value. The answer has the same IP, but the polarity feature is assigned a value by the focused answer particle. The IP can always be deleted under identity with the IP of the question, leaving just the focused particle to be spelled out. The Taiwanese yes-no questions discussed always have a polarity value. The focused particle in the answer either maintains the value, if the particle is affirmative, or changes the value, if the particle is negative. In the latter case, the IP cannot be deleted, as it will not meet the identity conditions on deletion. An additional crucial difference between English and Taiwanese is that the negative particle in Taiwanese always negates the polarity value of the IP it merges with, while the English negative particle can agree with a negative IP.