Previous research has shown that children go through a stage in which they know that the number words each refer to a distinct numerosity, yet do not know WHICH numerosity each number word picks out (Wynn, 1992). How do children attain this level of knowledge? We explore the possibility that particular properties of how number words are used within sentences inform children of the semantic class to which they belong. An analysis of transcripts of the spontaneous speech of three one- and two-year-old children and their parents (from the CHILDES database; MacWhinney & Snow, 1990) suggests that the relevant cues are available as input in parents' speech to children, and that children generally honour these properties of number words in their own speech. Implications of this proposal for word learning more generally are discussed.