The term acid rain describes rain, snow, or fog that is more acidic than normal precipitation. To understand what acid rain is, it is first necessary to know what an acid is. Acids can be defined as substances that produce hydrogen ions (H+), when dissolved in water. Scientists indicate how acidic a substance is by a set of numbers called the pH scale. The lower the pH number of the substance, the more acidic the substance is. Acid rain forms when pollutants produced by cars and factories enter the atmosphere and become dissolved in the water found in air. Acid rain can harm trees, soil, fish, and other living things. Acid rain has damaged many forests and lakes in Canada and the northeast United States. Acid rain can also damage manmade structures over time. Many statues and buildings around the world are made of a type of rock called limestone, which is easily damaged by acid rain that wears the stone away. This article provides a class activity which students can perform to observe the effect that acid can have on statues and buildings.