This article presents a class activity called "The Case of the Disappearing Snack" that involved second- and third-grade students. In this activity, the students investigated who stole the class snack. As they investigated the "crime," they developed critical-thinking skills and practiced using a microscope. Although microscopic evidence was not actually needed to deduce who committed the crime, using the microscopes added a "serious" element to the mystery that students really enjoyed. It took about 20 minutes for the students to observe all the clues and successfully determine the culprit. Including the introduction and explanation of the activity, the whole experience took about 30 minutes. The activity was a great way to help students hone their reasoning skills and feel comfortable with microscopes, a tool they will use often as they progress to more in-depth science explorations. (Contains 2 figures and lists 1 resource.)