We analyze the effects of a school-based program that offers children an opportunity to win prizes if they walk or bike to school during prize periods. We use daily child-level data and individual fixed effects models to measure the effect of the prizes, with variation in the timing of prize periods across different schools allowing us to estimate models with calendar-date fixed effects to control for day-specific attributes, such as weather and proximity to holidays. On average, we find that being in a prize period increases the riding behavior of participating children by sixteen percent, a large impact given that the prize value is just six cents per student. We also find that winning a prize lottery has a positive impact on ridership over subsequent weeks; consider heterogeneity across prize type, gender, age, and calendar month; and explore differential effects on the intensive versus extensive margins. (Contains 2 figures and 5 tables.)