This column focuses on promoting learning through assessment. This month's issue uses concept cartoons to assess students' ideas about the moon. Concept cartoons, formative assessment tools that reveal students' preconceptions and probe for conceptual understanding, have recently become popular in the United States, with teachers developing their own versions. "When Is the Next Full Moon?" is an elementary science probe with a concept cartoon format. This science probe provides an engaging, visual stimulus for eliciting students' ideas. The K-2 Earth and space science "Next Generation Science Standards" (NGSS) include the disciplinary core idea that patterns of the motion of the sun, moon, and stars in the sky can be observed, described, and predicted. "When Is the Next Full Moon?" is designed to address one component of this core idea-observing, describing, and predicting the monthly pattern of Moon phases. This science probe explores how the concept cartoon format helps students develop this important understanding and prepares them to meet the NGSS grade 1 performance expectation 1-ESS1-1.