Assessment serves many purposes in the elementary classroom. Formative assessment, often called assessment for learning, is characterized by its primary purpose--promoting learning. It takes place both formally and informally, is embedded in various stages of an instructional cycle, informs the teacher about appropriate next steps for instruction, and engages students in thinking about their own ideas. Formative assessment can take many forms. One form that has been used successfully in science education is the formative assessment probe. The "Uncovering Student Ideas in Science" series published by NSTA provides science educators with an extensive bank of formative assessment probes. This article features a probe and describes how elementary science teachers can use it to build their formative assessment repertoire and improve teaching and learning in the elementary science classroom. The author describes the formative assessment probe "How Far Did It Go?" in "Uncovering Student Ideas in Physical Science: 45 Force and Motion Assessment Probes" (Keeley and Harrington 2010), which can be used to reveal whether students recognize that units of distance traveled must be measured from the starting point to the ending point. It is especially useful in determining how students measure length when there is a nonzero origin. (Contains 1 figure.)