When sulfur dioxide and nitrogen oxide enter the air, it is most likely caused by the burning of fossil fuels from factories, automobiles, and homes. As these two chemicals rise into the clouds, they mix with moisture and become sulfuric acid and nitric acid. After traveling a long way through the atmosphere, the acids and moisture fall to earth as acid rain. Although acid rain is thought to be the only way of acids falling to earth, acids can actually form into acid snow, hail, sleet, fog, and even dry particles. All of these liquids carrying moisture can do a great deal of harming plants, animals, buildings, water sources, and even humans. The acid rain will soon evaporate into the clouds, repeating its harmful journey.