Critical thinking is the cornerstone of any undergraduate education. One technique for helping students develop critical thinking skills is argument mapping. Argument mapping involves diagraming the arguments for and against a proposition. The map also includes each piece of evidence used to support these arguments. Argument mapping can help students avoid confirmation bias by bringing attention to arguments and evidence which may run counter to their prior beliefs. It also prompts consideration of the strength of arguments and the quality of evidence. We describe how argument mapping can be incorporated in an undergraduate class and then present an example of a classroom exercise. The exercise involves mapping the arguments in a speech by American Secretary of State Colin Powell justifying the use of military force in Iraq before the United Nations Security Council in February 2013.