Basic qualitative research is presented in order to value and compare preschool (5-6yearold) and first-year primary level(6-7yearold) children's ideas in a Spanish school about several topics: waste, biodegradability, recycling, water pollution and water saving. To obtain this information, complementary instruments were used, although here the obtained results of the first instrument are reported. The results led to identifying the children's main responses on the topic as well as establishing some differences between the two educational stages. It was possible to establish progressive levels of categories for some of the ideas studied. Some teaching suggestions are made: it is ineffective to focus on what types of materials are deposited in each type of rubbish container when they cannot yet differentiate between the types of materials and their properties. Instead, a simplified approach should be developed perhaps by way of stories. A proposal could be developed on the biodegradability of materials as a visually distinguishable property, with some hands-on activities. We would also suggest providing specific examples as specific types of contamination, such as water pollution, on the living conditions of specific forms of life.