When predicting or explaining another person’s actions, weoften appeal to the physical effort they require; a person whoworks hard for something, for instance, must really like it (Liu,Ullman, Tenenbaum, & Spelke, 2017). But people are notonly motivated to avoid physical effort; they also seek to avoidmental effort (Shenhav et al., 2017; Kool & Botvinick, 2018).Here, we ask whether mental effort enters into preschoolers’understanding of other people’s actions. Across 4 experiments(N=112), we presented 4- and 5-year-old children with anagent (naive in Exp 1, 2 and 4, and knowledgeable in Exp 3)who can either move through a simple or complex maze envi-ronment with a specific goal (in Exp 1-3, to reach a play struc-ture beyond the mazes, and in Exp 4, to practice solving themazes). We found that children were sensitive to the physicaland mental effort associated with more complex mazes, and tothe trade-offs between effort and gain in skill. The intuitionthat choices impose costs on our bodies and minds appears toguide children’s understanding of other people.