We show that grit, a skill that has been shown to be highly predictive of achievement, is malleable in childhood and can be fostered in the classroom environment. We evaluate a randomized educational intervention implemented in two independent elementary school samples. Outcomes are measured via a novel incentivized real effort task and performance in standardized tests. We find that treated students are more likely to exert effort to accumulate task-specific ability, and hence, more likely to succeed. In a follow up 2.5 years after the intervention, we estimate an effect of about 0.2 standard deviations on a standardized math test.