In order to investigate the development of word-formation abilities, 3-, 5-, and 7-year-olds were asked to act out with toys, judge, and make up sentences containing instances of class extension. Some sample sentences are "Can you upside-down the clown?" and "Broom the spoon." Children dealt with such sentences in much the same way they deal with normal sentences. All ages were able to understand many of the sentences, 3-year-olds frequently made word-order errors in the comprehension task, and 5-year-olds often confused syntax with semantics in the judgment task. Particular lexical items and analogy types were difficult for all ages. The results suggest that word-formation abilities keep pace with syntactic development and are generative. (Author/CFM)