Recent research has suggested that rapid world learning develops within specific categorical domains as the result of previous within the domain (Thom & Sandhofer, 2009). The current studies further examined the relationship between childrens previous experience learning words within a category and their ability to extend additional words within the category. In Study 1, childrens extension of novel labels was compared across three common categories in relation to their existing vocabulary size within that category. In Study 2, children were trained in a greater or fewer number of category exemplars in two common categories, then tested in their ability to generalize new labels within each of these categories. Preliminary results indicate that greater experience learning words within a category predicts better extension within that category, but not in other categories, suggesting the development of rapid word learning is domain-specific, and occurs as the result of experience learning words within each category.