How do children use questions as tools to acquire new knowledge? The current experiment examined preschool children's ability to direct questions to appropriate sources to acquire knowledge. Fifty preschoolers engaged in a task that entailed asking questions to discover which special key would open a box that contained a prize. Children solved simple and complex problems by questioning two puppet experts who knew about separate features of each key. Results indicate dramatic developmental differences in the efficiency and efficacy of children's questions. Although even 3-year-olds asked questions, their questions were largely ineffective and directed toward inappropriate sources. Four-year-olds directed questions toward the appropriate sources but asked approximately equal numbers of effective and ineffective questions. Only 5-year-olds both asked the appropriate sources and formulated effective questions. Implications for the development of problem-solving abilities are discussed. (Contains 2 tables, 2 figures and 1 footnote.)