Preschool-aged children develop awareness of the words they do and do not know. Awareness of one’s lexicon may encourage word learning if children pay more attention to the definition of unknown words. Here, we tested 3-4-year-old children (N = 91) on a word learning task embedded in an e-book. When a novel word was read, children were either asked if they knew the word, asked a question about the storyline, or asked no question. Then they were given a description without visual input and asked to identify the referent’s picture from three choices. Participants who were asked if they knew a word before being provided with the definition identified more referents than children in the other conditions. Children’s word learning was predicted by short-term memory.