Emotional expressions are abundant in children's lives. What role do they play in children's causal inference and exploration? This study investigates whether preschool-aged children use others' emotional expressions to infer the presence of unknown causal functions and guide their exploration accordingly. Children (age: 3.0-4.9; N = 112, the United States) learned about one salient causal function of a novel toy and then saw an adult play with it. Children explored the toy more when the adult expressed surprise than when she expressed happiness (Experiment 1), but only when the adult already knew about the toy's salient function (Experiment 2). These results suggest that children consider others' knowledge and selectively interpret others' surprise as "vicarious prediction error" to guide their own exploration.