In spite of increased interest in game-based learning, relatively little is known about the learning process in games. The aim of this article is to study the process of conceptual change in the context of game-based learning in order to reveal playing behavior that triggers and facilitates it. Primary school pupils (N = 401) played a game of mathematics, in which they taught a virtual pet, a teachable agent that can reason based on how it is taught. Two experimental groups were formed: one group was tested under laboratory settings and one group worked in a natural context and was observed only virtually. The results of the study support existing theoretical models about conceptual change by indicating that some of the learners were more sensitive to the perception of cognitive conflicts and experienced them as solvable. Sufficient playing time, high motivation, game elements that triggered reflection, and good metacognitive strategies facilitated conceptual change. The results clearly indicated that educational games cannot be only supplementary amusement, but pedagogically well designed tools that should facilitate reflective processes both during and after the playing sessions. (Contains 2 tables and 6 figures.)