Only a small amount of fresh water is available for use. Water is treated or cleaned at filtration plants. People's actions affect the quality and availability of water. Addison Wesley focuses on insects and spiders in this unit. Scientists have developed a universal classification system based on animal's physical characteristics.
Insects have six jointed legs and three body parts.
Spiders have eight jointed legs and two body parts. Insects can affect other living things in helpful or harmful ways. The universe contains many types of stars. Groups of stars form constellations. The sun is an average sized star and is the center of our solar system.