This study examined the effect of scaffolding students who learned by designing assignments for a computer simulation on the physics topic of alternating circuits. We compared the students' assignments and the knowledge acquired in a scaffolded group (N=23) and a non-scaffolded group (N=19). The scaffold consisted of a Design Sheet that guided students through the different steps in the design of assignments (generate an idea, transform the idea into an assignment, and evaluate the assignment) and provided them with specific directions on how to perform these steps. On average, students in the non-scaffolded group designed more assignments than students in the scaffolded group, but the scaffolded students designed relative by more assignments about the relations in the domain, more often gave exact descriptions of the relations in the domain, and more often referred back to the computer simulation to explain their findings. No differences on knowledge tests, however, were found between the two groups of students. In the discussion, we give suggestions on how to adapt the scaffolding to improve not only the learning process but also knowledge acquisition.