The primary purpose of this study was to determine whether providing 6th-grade students with cooperative elaborative interrogation instruction would facilitate learning relative to providing them with cooperative learning, elaborative interrogation or reading-for-understanding instructions. All students were presented with 36 factual statements about six animals. Cooperative elaborative interrogation students were instructed to work collaboratively and use their prior knowledge to state why each fact is true. Cooperative learning students were told to work collaboratively to learn target materials, while elaborative interrogation students were instructed to generate answers to the why questions on their own. Reading-control students were also on their own and instructed to read the animal facts for understanding. For immediate free recall and immediate associative matching tests, students in the experimental conditions outperformed those in the control condition. Cooperative elaborative interrogation and elaborative interrogation students maintained f his advantage on a 30-day follow-up associative matching test, with elaborative interrogation students maintaining a significant advantage relative to reading controls on a 60-day associative matching follow-up. (There was also a strong trend favouring the cooperative elaborative interrogation condition on this 60-day measure.) The quality of the 'why' answer affected learning: Generating and listening to scientifically correct answers that used relevant prtor knowledge to clarify target information was associated with better memory for facts than were other types of study responses. Students in this study learned the most when they were explicitly directed to activate relevant prior knowledge that supports and clarifies new information-processing that occurs following either small-group or individual elaborative interrogation instruction.