Sketching and prototyping are parts of a 'reflective conversation with materials of a design situation' (Schön, 1992). To support this conversation, we developed a reflective tool -the Reflection Canvas- that facilitates reflection activities through sketching and prototyping on the one hand and verbalisation on the other. We introduced the reflective tool to design students. Based on observation and answers from a questionnaire data reveal that guided reflection structured the process in a helpful way. It also turned out students had difficulties to switch from visualisation to verbalisation.