Two-phase instructional designs such as Productive and Vicarious Failure employ preparatory activities before explicit instruction. It is assumed that during this preparatory activity, students need to activate their prior knowledge in order to be prepared for subsequent instruction. However, empirical findings on this preparatory mechanism are scarce, and causal evidence is lacking. In an experimental design (N = 165), we manipulated secondary school students' relevant prior knowledge activation by systematically varying the coverage of conceptual components of the targeted concept in solution attempts that students study in the preparatory phase: Students received solution attempts with either a high or a low coverage of conceptual components. The results reveal that students do not necessarily have to activate relevant prior knowledge covering all aspects of the targeted concept prior to instruction. Activating some relevant knowledge can prepare students for subsequent instruction and benefit students of all levels of prior knowledge. Moreover, we provide first evidence that students' prior knowledge activation becomes visible in their intermediate knowledge.