Presenter notes: Open with a claim that sounds impossible. A racing car goes flat out for a full lap and its velocity is zero. Say it flatly and wait, because somebody will object that it was obviously moving. Agree completely. Tell them it was moving very fast indeed, its speed was forty metres per second, and its velocity for that lap was still zero. Then say that today is the easiest lesson in the unit provided they noticed one thing yesterday, and ask them what displacement was for a walk that returned to its starting point. Zero. Half the class will get there before you finish the sentence.