Who hasn’t seen a dramatic flash of lightning, only to hear the dramatic “crack” of thunder several seconds later? But why does the thunder reach our ears after we see the lightning? Or, why does the sound of a high-flying jet airplane passing overhead seem to originate far behind the plane itself? The sound and the sight of a distant, loud event are said to be out of phase, that is, they aren’t experienced simultaneously. Again, why? In this activity, students will collect data and determine the reason for this phenomenon by calculating the speed of sound and comparing it to the speed of light. In the activity, which can be undertaken with simple materials on any playground or large outdoor area, students will measure, convert units, compare, and reach empirical conclusions based on their own investigation of the phenomenon.