In this study, we investigated one student as she engaged in the exploration of a graph theory problem, the Graceful Tree Conjecture. The study took place in a classroom in the Midwestern United States which contained eighteen students. We report on a descriptive case study of one student, Heidi, as she works her way through different classes of tree graphs and advances through different types of reasoning, justifications, and proving a select class of tree graphs can be labeled gracefully. We found that with repeated experiences, Heidi, was able to advance her level of reasoning and creations of justifications. [For the complete proceedings, see ED606556.]