Shared book reading, and the conversation that accompanies it, can facilitate young children's vocabulary growth. To identify the features of extratextual questions that help 3-year-olds learn unfamiliar words during shared book reading, two experiments explored the impact of cognitive demand level, placement, and an approximation to scaffolding. Asking questions about target words improved children's comprehension and production of word-referent associations, and children with larger vocabularies learned more than children with smaller vocabularies. Neither the demand level nor placement of questions differentially affected word learning. However, an approximation to scaffolding, in which adults asked low demand questions when words first appeared and high demand questions later, did facilitate children's deeper understanding of word meanings as assessed with a definition task. These results are unique in experimentally demonstrating the value for word learning of shifting from less to more challenging input over time. Discussion focuses on why a scaffolding-like procedure improves children's acquisition of elaborated word meanings. (Contains 4 tables.)