What is a scientific explanation and how can we support students in explaining changes in matter? The Fizzy Antacid task is intended to elicit and develop preservice teachers’ CKT related to students’ scientific explanations of changes in matter. The first part of the task asks preservice teachers to develop their own explanations and engage in scientific argumentation, while considering how elementary students might explain the Fizzy Antacid phenomenon. Student Explanations: Fizzy Antacid and Evaluating Explanations: Fizzy Antacid are included to help preservice teachers analyze student scientific explanations and strategize ways to help them strengthen their arguments. The packet contains tasks, an elaborated answer key, lesson plans, reading pages, and other resources to support preservice teachers in helping elementary students develop explanations of changes in matter.