This study investigates how, and to what extent, young readers (7-8 year-olds) use text information or their prior knowledge when answering comprehension questions about narrative texts. The children were asked to explain how they found out their responses by answering the following question: "how do you know this answer?" Their answers and justifications were analysed both qualitatively and quantitatively. The text proved to be the main source of information for these readers. However, sometimes the children seemed to ignore the text and over rely on their prior knowledge to answer the questions. The procedure of asking children to justify their answers was shown to be a good way of specifying more precisely some of their problems in text comprehension. It also seemed to encourage them to look back at the text and review their responses and, as such, it could be considered a useful tool to improve children's reading comprehension.