Students taking Operating Systems encounter many different policies and algorithms that they need to learn and understand. Two important topics are CPU scheduling and virtual-memory page replacement, each of which have about five example policies. Traditionally, we have used paper-based worksheets to support student understanding, but this approach limits the opportunity for repetitions to the number prepared by the instructor and direct feedback requires instructor support. Previous instructors report using simulations, together with informal assessments of those tools, but many of these simulations have been lost to the ravages of time. In response, we have built GraySim, which contains two simulations: one that supports CPU scheduling and another that supports virtual-memory page replacement. Operating Systems students can practice their understanding of the various policies, receiving both general and policy-specific feedback along the way. In this session, we demonstrate how each is used, show common student errors and the resulting feedback, and give the audience an opportunity to try them out. We invite those interested to adopt the simulations and collaborate to do a broader evaluation of student learning outcomes.