This study examines the mechanisms underlying long-run reductions in energy consumption caused by a widely studied social nudge. Our investigation considers two channels: physical capital in the home and habit formation in the household. Using data from 38 natural field experiments, we isolate the role of physical capital by comparing treatment and control homes after the original household moves, which ends treatment. We find 35 to 55 percent of the reductions persist once treatment ends and show this is consonant with the physical capital channel. Methodologically, our findings have important implications for the design and assessment of behavioral interventions.