Presenter notes: Open with a problem rather than a definition. Tell the class you have hidden something under one desk and you are going to describe where it is using one number only. Say it is four away. Let them try, and let them fail, and let somebody complain that four away could be in any direction. That complaint is the lesson. Then say four along and two back, and somebody finds it immediately. Two numbers, one for each direction, and they discovered why before you named anything.