This article describes how observing whether objects sink or float in water using the P-E-O (Predict, Explain, and Observe) technique is an elementary precursor to developing explanations in later grades that involve an understanding of density and buoyancy. Beginning as early as preschool, elementary students engage in activities that encourage them to predict whether an object will sink or float when placed in water, and then test their predictions by making observations. Children often base their predictions about whether an object or material will float or sink on the observable properties of the object they are testing such as its size, felt weight, heaviness for its size, or shape. Therefore it is important for elementary students to have multiple experiences describing the properties of objects and materials, predicting whether they will float or sink, supporting their predictions with explanations that use the properties of the objects or materials as evidence, and testing their predictions. What makes the P-E-O strategy an important technique for formative assessment is that it provides teachers with insights into the misconceptions students hold prior to engaging in inquiry that are then used to facilitate the learning process. It also promotes learning through a process of conceptual change that involves important scientific practices.