The ability to categorize problems is a measure of expertise in a domain. In order to help students learn effectively, instructors and teaching assistants (TAs) should have pedagogical content knowledge. They must be aware of the prior knowledge of students they are teaching, consider the difficulty of the problems from students' perspective and design instruction that builds on what students already know. Here, we discuss the response of graduate students enrolled in a TA training course to categorization tasks in which they were asked to group problems based upon similarity of solution first from their own perspective, and later from the perspective of introductory physics students. Many graduate students performed an expert-like categorization of introductory physics problems. However, when asked to categorize the same problems from the perspective of introductory students, many graduate students expressed dismay, claiming that the task was impossible, pointless and had no relevance to their TA duties. We will discuss how categorization can be a useful tool for scaffolding and improving pedagogical content knowledge of teaching assistants and instructors.