This paper discusses how modifying the true-or-false (T/F) test, by requiring written explanations, reveals major weaknesses and hazards of the simple (unexplained) T/F tests popular among teachers. But, more important, the modification transforms the test into a very efficient formative assessment tool. It opens a window that allows a teacher to track how students think as they analyze questions. Yes, it takes up more time--for the students as well as the teacher. But being able to follow students' thought processes, and identify where things might be going wrong, can be immensely illuminating! For both the students and the teacher. While the examples presented may be specific to economics, the basic ideas and conclusions raised here may apply to other fields of study as well.