A variety of tools have been created to understand student performance on multiple-choice tests, including analysis of normalized gain, item response curves, and more. These methods typically focus on correct answers. Many incorrect responses contain value and can be used as building blocks for instruction, but present tools do not account for productive reasoning leading to an incorrect response. Inspired by Item Response Curves, we introduce Idea Use Curves, which relate frequency with which an idea is used to student performance. We use this tool to consider ideas which may be present in both correct responses and distractors, letting us attend more to students' conceptual understanding. This tool is made with the goal of identifying ideas that are consistently used by students who perform well or poorly, allowing researchers and instructors to look beyond the "correct/incorrect" paradigm. We explore student reasoning about energy as a proof of concept for this method.