Focusing on words such as bag, hammer, kiss, and dance, which are subject to functional shift, i.e. alternate between noun and verb, this article argues against the traditional view that a category-changing rule derives verbs from nouns and vice versa. The alternative proposal is that root lexemes in general, and words like these in particular, are semantically underspecified with respect to the noun/verb distinction. The lexical semantic representations of such words include event schemas that are compatible with either noun or verb meanings. The verb vs. noun aspect of the meanings is supplied by the morphosyntactic contexts in which they appear. This analysis is shown to account straightforwardly for the general properties of functional shift, such as its ubiquity, productivity, and semantic regularity, and to be supported by both standard kinds of distributional evidence and neurolinguistic evidence.