Acid rain is any form of precipitation with high levels of nitric and sulfuric acids. It can occur in the form of snow, fog, and even dry materials that settle to earth. Most acid rain is caused by human activities. When people burn fossil fuels, sulfur dioxide and nitrogen oxides are released into the atmosphere. These gases react with water oxygen and other substances to form sulfuric and nitric acid. Winds may spread these acidic solutions over hundreds of miles. After it falls to earth, acid rain enters water systems as runoff and sinks into the ground. This can make water toxic to prey, fish clams, fish, and other aquatic animals. The rest of the food chain, including non-aquatic species such as birds, is often affected as well as. Acid drain also harms forests by damaging trees leaves, robbing the soil of essential nutrients and making it hard for trees to take up water. By designing cleaner power plants and using fewer fossil fuels, we can reduce the number of pollutants that create acid rain.