What ideas do students have about small particles of matter? The Two Cups of Liquid task is intended to elicit preservice teachers’ CKT related to students’ ideas about the small particle model (SPM) for matter. The first part of the task asks preservice teachers to articulate their own understanding about why two cups containing different volumes of liquid might have the same mass, while the second examines how elementary students might respond. Preservice teachers engage in anticipating and understanding students’ ideas, including building an awareness of common student conceptions, as well as evaluating the scientific accuracy of students’ ideas. The associated resources and activities in this packet, including a set of sample teacher responses, are designed to support teacher educators in developing preservice teachers’ CKT through understanding and interpreting a range of possible student ideas about the small particle model (SPM) of matter. This packet contains tasks, lessons, and resources for supporting preservice teachers.