In this activity, learners explore the relationship between cooling water vapor and condensation. Learners investigate condensation forming on the outside of a cold cup. To see if the condensed water vapor comes from the air, learners use two cold cups, but limit the air around one of them by placing it in a plastic bag. Learners then compare the amount of moisture that forms on the outside of both cups. They will see that more moisture forms on the exposed cup. Learners will conclude that water vapor from the air must condense to form the moisture on the outside of a cold cup. Use this activity as a followup to "Condensation," an activity from the same resource (see related resources).