The focus of this study was the strategies used by young children between 18 and 42 months for correcting the errors they made as they attempted to nest a set of 5 seriated cups. In the process of combining the cups, the children committed numerous errors (such as putting a cup that was too large on a smaller cup), and they tried to correct the majority of those errors. Detailed examination of the children’s correction attempts revealed that the strategies they used changed substantially with age, becoming increasingly more flexible and involving more extensive restructuring of the relations among the cups. Earlier correction attempts tended to focus on a single, nonfitting cup or on a single relation between 2 cups. Later-appearing strategies involved the coordination of relations involving several cups. The same trend toward increasing flexibility of thought and action also appeared in the procedures the children used to combine the cups. This study thus documents a finely graded series of cognitively significant changes in children’s constructive activity during a period that has been poorly differentiated by cognitive developmental research. In so doing, it demonstrates the usefulness for problem-solving research of analyzing how subjects go about trying to rectify their own mistakes.