This study explores the progression from student justification to generalization in the course of example-based reasoning. Data was collected through group interviews with high school students who were working collaboratively on a task of determining connections between perimeter and area of tile shaped patterns. The task called for making and justifying conjectures regarding patterns of specific number of tiles. Our findings show that the task elicited collaborative example-based reasoning that evoked spontaneous generalizations about patterns of any number of tiles. The findings point to the importance of collaboration in generalizing, as well as to the intuitive nature of generalizations. [For the complete proceedings, see ED606531.]