Educational simulations and serious games hold great potential for creating engaging and productive learning environments in STEM domains. In this paper we present and reflect on some of our research findings in a series of studies with a computer simulation in the domain electricity. These studies used the same simulation with varying instructional designs and over a range of grades. Interestingly, each design has always had a unique influence on student performance and/or engagement. We hope our results can provide some new insights for designers when designing simulations (or, serious games) for education and for educators utilizing these designs in practical settings.