Interactive computer simulations with complex representations and sophisticated graphics are a relatively new addition to the classroom, and research in this area is limited. We have conducted over 200 individual student interviews during which the students described what they were thinking as they interacted with simulations. These interviews were conducted as part of the research and design of simulations for the Physics Education Technology (PhET) project. PhET is an ongoing project that has developed over 60 simulations for use in teaching physics, chemistry, and physical science. These interviews are a rich source of information about how students interact with computer simulations and what makes an educationally effective simulation. The interviews demonstrate that the simulation must function intuitively or the student's attention is focused on how to use the simulation rather than on the topic presented. Here we provide guidelines for intuitive interface design developed by this research. These cover layout, tool use, help, and representations that we use to create a simulation. We give examples from interviews, which demonstrate the effectiveness of each guideline for engaging students in educationally productive interactions.