Three experiments investigated the processes by which 2-year-olds acquire the language to express category hierarchies. The first experiment studied how children use current linguistic knowledge to constrain the potential meanings of new words. This experiment compared interpretations of new words given to objects the children could already name with their interpretations of new words given to novel objects. In the second experiment, the children were taught new words for familiar objects, with the objective of learning why children use a new word for a known object to apply only to the object explicitly named. The third experiment further explored the results of the second, investigating whether children interpreted a new word for a familiar object as a subordinate or contrasting basic category in comparison with known words. The experiments demonstrated that 2-year-old children interpret a new word differently depending on whether they already know a word for the object being named. In addition, the results suggest that 2-year-olds interpret a new noun applied to a familiar object as a subordinate category term, a surprising result because older children have been found to have difficulty with hierarchical relations. (MSE)