This paper explains how to use a new software tool for argument diagramming available free on the Internet, showing especially how it can be used in the classroom to enhance critical thinking in philosophy. The user loads a text file containing an argument into a box on the computer interface, and then creates an argument diagram by dragging lines (representing inferences) from one node (proposition) to another. A key feature is the support for argumentation schemes, common patterns of defeasible reasoning historically know as topics (topoi). Several examples are presented, as well as the results of an experiment in using the system with students in a university classroom. © Teaching Philosophy, 2006.