Interactive science simulations (sims) have become popular tools for science educators, and research confirms that sims can improve student learning (Rutten, van Joolingen, and van der Veen 2012). Over the past 15 years, the PhET Interac- tive Simulations project at the University of Colorado Boulder has created a collection of 150 open (free) educational sims for teaching science and math. It has been exciting to see teachers bring these resources into their classrooms, now with over 100 million uses per year.