The core idea of this paper is familiar to teachers: While a child is reading, explain unfamiliar words. Project LISTEN’s Reading Tutor listens to children read aloud and helps them learn to read. We want the Reading Tutor to explain unfamiliar words. To elicit explanations from an expert, the computer should suggest -- or let the expert select -- words to annotate. To capture explanations, the expert will type in and then narrate an explanation. Text and narration will be saved for later use. To utilize explanations during assisted reading, we will display the explanations as extra sentences to be read aloud with the computer’s help. Explanations will be provided on student request or computer tutor initiative. We focus here on how to select words for annotation.