Professional development (PD) can support science teachers to learn about instructional reforms, but more work needs to be done on broadening our understanding of how specific PD activities support teacher learning. One understudied PD activity is the "student hat": when teachers engage in student learning activities while considering ideas, language, and feelings students might have to build their empathy for how student experience reform instruction. Little is known about if and how student hat activities support teacher learning. I conducted a single embedded case study of a 2.5-day PD for middle school science teachers using the OpenSciEd curriculum. I interviewed 12 participants to understand how they perceived the student hat activities and analyzed 36 hours of PD video, focusing specifically on moments in which participants struggled to act in student hat, to gain insights on how it helped them to learn. Teachers found student hat difficult, but it helped them better understand science ideas, their students, and the instructional approach. These learning outcomes likely occurred because of two mechanisms: creating a safe environment to be wrong and building epistemic empathy with students. By allowing teachers to feel safe expressing confusion with content ideas, the student hat helped teachers to build their science understanding. Developing teachers' epistemic empathy for students helped them to understand how students might think and feel while engaging in reform instruction.