One of the most important concepts being taught in principles classes is the idea of "thinking on the margin." It can also be one of the most difficult to get across. One of the most telling examples, according to this author, comes in trying to get students to learn the profit maximizing condition for perfectly competitive firms. She has tried a variety of methods to get students to see why they should look for the quantity where price is equal to marginal cost, but have frequently been disappointed at the significant number of students that do not remember this condition. She writes in this article that she often observes students trying to compute profit (even when they don't have enough information to do so) instead of computing marginal costs. This article describes a short classroom experiment that Bangs paired with a homework assignment that has been useful in her classes in getting students to see the value of working with marginal costs.