Modeling requires that people develop representations or procedures to address particular problem situations (Lesh et al. 2000). Mathematical modeling is used to describe essential characteristics of a phenomenon or a situation that one intends to study in the real world through building mathematical objects. This article describes how fifth-grade students explored a modeling experience through the Mysterious Tool activity. Specifically, the aim of the activity was to find out the relationships among weight, distance, and balance by exploring a lever that students encountered for the first time. Condensed qualitative results of a study that documented students' thinking are presented, as well, to support the investigation.