Middle school students make great learning gains when they participate in lessons that invite them to practice their developing scientific reasoning skills; however, designing developmentally appropriate, clear, and structured lessons about scientific thinking and reasoning can be difficult. This challenge can be met through lessons that teach students not only what to know, but how to think about what they know. The lesson-planning model introduced in this article, called the Thinking Lesson Model, highlights two specific types of scientific reasoning processes: inductive thinking and deductive thinking.