Some topics in an environmental science class are core to many different areas of the subject. Without a firm knowledge of these topics, one will have a tough time understanding many other topics. For instance, without a firm knowledge of chemistry and how chemicals move within an ecosystem, one cannot fully appreciate why toxins are harmful, why burning fossil fuels in large quantities is a bad idea, or how the hole in the stratospheric ozone layer formed. The activities below cover some of these basic issues.