We investigated whether specific input helps 3-1/2-year-olds discover that the last word in a count represents its cardinal value (i.e., the cardinal word principle). In Study 1, we contrasted four training approaches. The only approach to yield significant improvement was to label a set's cardinality and then immediately count it. This training is consistent with previously hypothesized mechanisms based on juxtaposing a set's cardinal label with its count in close temporal contiguity (e.g., Klahr & Wallace, 1976; Schaeffer, Eggleston, & Scott, 1974), as well as general theories of comparison and categorization (e.g., Gentner, 2005). In Study 2, we asked parents to read picture books to their preschool children and found that they rarely provide cardinal labels immediately followed by counting, even when asked to read a book about number. (Contains 3 figures and 3 tables.)