This research explored the development of one insight about the mind, namely, the belief that people's trains of thought differ even when they see the same stimulus. In Study 1, 5-year-olds, 9-year-olds, and adults heard stories about characters who saw the same object. Although the older groups predicted the object would trigger different trains of thought, most 5-year-olds did not. In Study 2, 5-year-olds (preschoolers and kindergartners) and 7-year-olds heard similar stories, plus stories with additional individuating information about each character. With age, children increasingly recognized that thoughts would differ and could explain why. The development of this insight during the school years likely provides children with a more complete understanding of what it means to be a unique individual.