Presenter notes: Open with a question that has an obvious wrong answer. Write 0.7 and 0.65 on the board and ask which is larger. A good number of the class will say 0.65 because it has more digits, and that is worth surfacing now rather than discovering in an exam. Then ask why money and measurements are always written as decimals rather than fractions. The answer to both questions is the same idea, and it takes about a minute to explain once the lesson gets going.