This column focuses on promoting learning through assessment. The author discusses the formative assessment probe "Pond Water," which reveals how elementary children will often apply what they know about animal structures to newly discovered microscopic organisms, connecting their knowledge of the familiar to the unfamiliar through overgeneralization. The formative assessment probe, "Pond Water," is designed to find out whether students use their knowledge of animal structure to overgeneralize about the internal structure of single-cell pond organisms. Results from the "Pond Water" probe showed similar ideas held by fifth-grade students. The most common response among 43 fourth- and fifth-grade students who participated in a field test of this friendly talk probe was the answer choice, "I think they have a few of the organs most animals have." (Contains 2 figures and 1 online resource.)