All greenhouses operate on the same basic principle. Radiant energy (light) from the sun can pass through transparent and semi-transparent materials. When the light arrives inside a closed space, it is absorbed by the surfaces within, then radiated again as thermal energy (heat). That energy is less able to pass through the transparent or semi-transparent materials, so the heat is trapped inside. Anyone who has entered a car parked in a sunny location knows what trapped heat feels like! This heat energy warms the air, enabling plant growth. As a simple but powerful exploration of this phenomenon, invite students to place a thermometer inside a clear, closed glass jar in the sun. Place a second thermometer next to the jar. After half an hour, compare the two temperatures. Your students may be surprised at the difference between the two readings.