Designers give themselves something to "react to" and they make it "rich." During design, what they react to can take many forms: a homepage wireframe, an Excel spreadsheet, building drawings, and a Tweet prototype. Using a phenomenological research design using an interactive methodology and multiple data collection methods, the researchers looked at designers reflecting-in-action. A research question was: What is the impact of reflection-in-action on evaluation processes while a design is in progress and not yet complete? Focusing specifically on a theme that emerged from this research question, this manuscript explores eight designers, who while reflecting-in-action, took stock in and reacted to external representations, which were rich in context, information, and constraints. It delves into the experiences that these multidisciplinary designers had in their respective design spaces reacting to and evaluating external representations. The researchers looked at how rich external representations intertwined in context, information, and constraints helped designers move to partial solutions that moved the design forward. The researchers discuss implications for students and teachers, and share a design experience where inexperienced designers (as university students) can practice taking stock in and reacting to external representations.