Presenter notes: Open with a challenge rather than a definition. Tell the class you are thinking of a triangle and you will answer questions about it, one at a time, and they have to work out how many questions they need before they could draw your exact triangle. Let them ask. After one answer they cannot draw it, after two they still cannot, and after three of the right kind they can. Somebody will ask for all three angles and get stuck, which is the best thing that can happen, because it sets up the AAA case for later. Then name the lesson: how many facts, and which facts, pin a triangle down.