Presenter notes: Open with two practical problems rather than two definitions, and do not name either idea yet. First: a floor 240 cm by 180 cm, and the largest square tile that fits with no cutting. Second: two buses leaving every 24 and 36 minutes, and when they next leave together. Ask the class which answer will be a small number and which will be a large one. They can tell without calculating anything, and that instinct is exactly what the whole lesson is going to formalise.