You eat acids all the time without knowing it and it's usually perfectly healthy. The danger is eating strong acids in high concentrations. The word "acid" describes a wide variety of substances that lie on a continuous spectrum of strength from essentially neutral to extremely acidic. As most chemistry textbooks note, we are surrounded by acids. Citric acid is what gives lemons, limes, oranges and grapefruits their zing. Phosphoric acid is used in colas. The building blocks of life; amino acids, fatty acids, DNA, and RNA; are all acids. Acid is one of the fundamental flavors our tongues can taste. We call it "sour" in everyday life. Anything that tastes sour – from vinegar (acetic acid) to sour cream (lactic acid) – has acid. Even strong acids can be safe in low concentrations. Hydrochloric acid is found naturally in the human stomach and is used to help digest food. The real danger to humans is when strong acids are found in high concentrations, such as the hydrochloric acid in toilet bowl cleaners or the sulfuric acid in car batteries. Returning to the original question, how do you know if a substance is an acid with dangerously high concentration? There are all sorts of tests you could do, but it's better just to play it safe. Never eat an item unless you are sure it is food. Generally, it's not a good idea to eat household cleaners, disinfectants, antiseptics, rust removers, or battery juice, as these contain toxic chemicals such as acids in high concentration.