Acquiring the correct meanings of number words (e.g., seven, forty-two) is challenging, as such words fail to describe salient properties of individuals or objects in their environment, referring rather to properties of sets of such objects or individuals. Understanding how children succeed in this task requires a precise understanding not only of the kinds of data children have available to them, but also of the character of the biases and expectations that they bring to the learning task. Previous research has revealed a critical role for language itself in how children acquire number word meanings, however attempts to pinpoint precisely the strong linguistic cues has proved challenging. We propose a novel “syntactic bootstrapping” hypothesis in which categorizing a novel word as a determiner leads to quantity-based interpretations. The results of a word learning task with 4 year olds indicates that this hypothesis is on the right track.