Young children are able to categorize animals on the basis of unobservable features such as shared biological properties (e.g., bones). For the most part, children learn about these properties through explicit verbalizations from others. The present study examined how such input impacts children's learning about the properties of categories. In a training study with thirty-six 2.5-year-olds (M[subscript age] = 2;9), we tested the prediction that a relatively small amount of input highlighting the importance of unobservable properties would lead to a gradual shift in children's use of these properties for categorization. Children with no initial categorization bias were trained to categorize either on the basis of shared unobservable properties or shared observable properties. During 3 days of training, children gradually developed a categorization bias in the direction of the property type for which they received training and sustained this bias more than a week after training. Also, a set of children who exhibited an initial perceptual bias showed an abrupt change in how they categorized after only 1 training session.