Presenter notes: Ask who has slipped over in the last year, and take three or four accounts. Then get the details rather than the story: was it wet or dry, smooth or rough, and were you on something that rolls? A pattern appears without you saying anything, and the class has just derived what friction depends on before hearing the word. Then set up the surprise you will spring later. Ask them to predict what happens if you drag a brick lying flat and then stand it on its edge. Almost everybody says much less force. Write the prediction on the board and leave it there. You will come back to it.