Presenter notes: Start with a list rather than a definition. Ask the class to call out everything sour they have eaten this week and fill the board: lemon, curd, tomato, amala, pickle, chuk, tamarind, cold drink. Then tell them that every single item up there contains an acid, and that they have been eating acids at every meal of their lives. Let the word acid stop sounding like something in a laboratory bottle. Then plant the question the lesson answers: if all of that is acid and all of it is safe, why does the book say some acids must never be eaten, and what exactly is the difference?