Understanding the meaning of rational numbers and how to perform mathematical operations with those numbers seems to be a perennial problem in the United States for both adults and children. Based on previous work, we hypothesized that giving students more time to practice using rational numbers in an environment that enticed them to apply their understanding might prove educationally beneficial. We developed a video game, based on two key ideas about addition and rational numbers, to investigate this hypothesis. We also analyzed the effects of different types of feedback provided to students during the videogame. Our findings in this initial study suggest that designing such a video game is not only possible, but also that students using a game designed in this manner can increase their ability to add rational numbers even when playing the game for a relatively short period of time. Since the effect size of a single 40-minute intervention is moderate, we discuss the need for future studies designed to spread game play over several class periods and to include instructional resources external to the game. We discuss implications for the larger efficacy study to follow. Appended are: (1) "Save Patch" Game Board; (2) Knowledge Specifications (Learning Objectives) in "Save Patch"; and (3) Pretest to Posttest Effect Sizes (Uncorrected for Correlation) by Intervention and Class Type. (Contains 15 tables.) [The work reported herein was supported under the Educational Research and Development Centers Program.]