Presenter notes: Open by holding up a protractor and a compass, one in each hand. Ask the class which one is cheating. Let them argue for a moment, then say it plainly: the protractor already has every answer printed on it, so using it to make an angle is copying. The compass has no numbers anywhere on it, and yet it can produce a perfect sixty degrees. Today we find out how something with no numbers on it can know what sixty degrees is. Tell them the protractor stays in the box until the last five minutes of the lesson, when it is used only to check. Two periods are allotted to this topic.