Effectively communicating designs to stakeholders or end users is a critical step in the design process yet can be a difficult challenge for engineers. Prototypes are unique tools that can enhance communication between these two groups, as prototypes are physical manifestations of the designer's mental model. Previous work has demonstrated that novice designers often struggle to use prototypes as communication tools. We argue that it is critical that engineering students learn to fully leverage prototypes, and thus the current work sought to understand the relationship between argumentation, prototyping, and design decisions. In order to understand the communication patterns of novice designers during a prototyping task, a controlled study was conducted with a total of 46 undergraduate engineering students. The analysis of quantitative and qualitative data point to the intricate linkages between how students make material decisions and how they justify those decisions.