"Substances that are acidic or basic make the eyes sting, so baby shampoo is made neutral. Acids are a very common group of chemical compounds, many of which occur naturally. Acids can be strong or weak. Citric acid, which occurs naturally in lemons, is a weak acid. Hydrochloric acid used for soldering and sulfuric acid used for battery acid are very strong acids. Bases, often called alkalis, are another group of chemical compounds that have different chemical properties from acids. When bases and acids are added together, they will neutralize each other's properties."