CCC Kei Wai Primary School (Ma Wan) is a 30-classroom 7-story primary school located at Ma Wan Island. The campus was completed in 2003. There are three arrays of photovoltaic modules installed on the roof with an expected annual yield of 5600 kWh a.c. electricity. This system is supported by a research project called "Building Integrated Photovoltaic (BIPV) School Design" funded by the Innovation and Technology Commissions of the Hong Kong Government with matching sponsorship from the China Light and Power Research Institute (HK). After moving into the school in 2003, the author began to plan solar energy activities for her students. This article describes a classroom activity where pupils construct a "greenhouse heater." The activity was run as a project-based competition for all Primary 5 classes with 3 to 4 pupils per group. Each group was asked to construct a small greenhouse with a container inside. During the competition, 10 ml of water was poured into the container. Then the greenhouse was placed under the Sun for 20 minutes. The rise in the temperature of the water was the indication of the performance of their greenhouse. Then pupils would present and explain their designs. The construction activity was to give a chance for the pupils to apply various investigation skills, rather than to teach them those skills. Skills needed for this activity were taught over 9 separate class periods described herein.