This article presents the "modus operandi" of a game, the so-called Game of Shapes, which can be used as an active learning tool to teach Constructivism to introductory International Relations courses. While sharing our own experience of the development and application of the game in a laboratory with International Relations students, this paper aims to provide a teaching template that can assist in the problematization of the statist and individualistic ontology of rationalist theories. Hence it is an interactive dynamic that helps to introduce and open up the discussion of the contingency of the social world, which is the base to understand any of the several and intricate tenets of Constructivism and any other critical approaches. Most of the students' feedbacks were very positive regarding their comprehension of the content presented. They provided important feedback on how the activity was valuable in order to grasp complex concepts, important to constructivist theories in International Relations.