Substances that are more dense than water will sink in water. The water cycle consists of evaporation, condensation, precipitation and accumulation. The sun is the source of energy that drives the water cycle. The earth has all the water it will ever have. 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.