What does it mean to help students develop a small particle model of matter? The Mystery Bubble task is intended to elicit preservice teachers’ CKT related to students’ ideas about the small particle model (SPM) for matter, particularly the idea of empty space between the particles. The task presents a phenomenon in which combining ethanol and water results in a mixture that occupies less space than the combined volume of each. The Modeling the Mystery Bubble task asks preservice teachers to consider teaching moves apprioriate to engaging students in the practice of modeling to explain the phenomena. The full packet contains tasks, an elaborated answer key, lesson plans, and other resources to support teacher educators in helping preservice elementary teachers develop CKT for engaging students in modeling and using representations of the small particle model of matter.