Preschool children, aged 4 and 5 years, report different understandings of the phenomenon of plant reproduction. We use their ideas as a starting point in designing an electronic device that illustrates phases of plant growth in response to children's augmentations or inhibitions of levels of water, light and temperature. Sensors are the crucial bridge between physical and virtual, as they detect physical actions and trigger virtual representations. The represented ``digital seed'' behaves much like a real seed, thus compressing the time required for phases of plant growth and providing a tool for children to experiment with relevant environmental conditions. We work with the children throughout the course of the project, in designing and refining the tool and in following conceptual changes as they occur through use of the device.