What cognitive demands foster understanding of thermodynamics for middle school science students? We successively modified the cognitive demands of a 13-week thermodynamics curriculum for four cohorts of 100-200 eighth graders while maintaining the same basic experiments and real-time data collection software. When comparing posttest performance across four versions, we found two- to fourfold increases in understanding when (a) students actively predicted outcomes and reconciled results, and (b) students used a heat-flow model of thermodynamics to integrate their experimental results. We argue that the curriculum must explicitly motivate students to construct understanding, and that middle school students benefit from what we call “pragmatic models” of scientific concepts.