Most students love to play games. Ernest (1986) believed that games could be used to teach mathematics effectively in four areas: motivation, concept development, reinforcement of skills, and practice of problem-solving strategies. Fifteen is an interesting and thought-provoking game that helps students learn mathematics at the same time. Playing Fifteen is like playing tic-tac-toe on a 3 x 3 magic square in which the three numbers in each row, column, and diagonal add up to 15. The game can be used in the classroom to motivate and help students learn mathematics and develop heuristic skills such as examining all possible scenarios and systematic listing (Polya 1957), spatial visualization (Presmeg 2008), and thinking skills, including predicting, conjecturing, generalizing, and checking (Ainley 1990). (Contains 10 figures.)