This paper focuses on developing students' informal ideas of inference and argumentative skills. This topic is of current interest to many researchers and teachers of statistics. We study fifth graders' learning processes in an exploratory interdisciplinary learning environment that usesTinkerPlots to scaffold and extend students' statistical reasoning. The careful design of the learning trajectory based on growing samples heuristics coupled with the unique features ofTinkerPlots were found instrumental in supporting students' multiplicative reasoning, aggregate reasoning, acknowledging the value of large samples, and accounting for variability. These processes were accompanied by greater ability to verbalize, explain and argue about data-based inferences. In the light of the analysis, a description of what it may mean to begin reasoning and arguing about inference by young students is proposed.