Presenter notes: Start with a nail, a length of string and a piece of chalk, and draw a large circle on the floor or the playground. Do it without saying anything first, then ask what kept the chalk in a circle. Somebody will say the string stayed the same length. That is the definition, discovered rather than announced, and everything in the lesson hangs off it. Keep the string and nail visible for the rest of the period, because you will point back at them when you introduce radius and centre.