Computational modeling and thinking skill sets were previously relegated to computer scientists and programmers. As a result, computational tools are largely unfamiliar to K-12 science teachers and students. Using Mathematical and Computational Thinking and Developing and Using Models were included in the "Next Generation Science Standards" ("NGSS") as core science and engineering practices (SEPs) (NRC 2012). The "NGSS" lays out a progression for Using Mathematical and Computational Thinking that, in elementary school, includes familiar activities such as making quantitative comparisons, creating graphs, and using computer simulations. By high school, students should advance and be able to create or revise simulations using algorithms--mathematical and computational tasks that are far less familiar to both students and teachers. The lesson presented in this article is designed to assist students and teachers to move from the familiar (simulation user) into less-explored territory (simulation creator). The lesson was implemented in a high school STEM elective course and consists of four 55-minute class sessions that help students develop greater proficiency in the two targeted SEPs. The lesson uses two open-source software tools: Concord Consortium and Insight Maker. Students first use a computational representation to illustrate the relationships among Earth systems and how those relationships are being modified due to human activity (HSESS3-6) and then (2) develop a model based on evidence to illustrate the relationship between systems or components of a system (HS-LS2-5).