To be a good teacher, you need some of the gifts of a good actor: you must be able to hold the attention and interest of your audience; you must be a clever speaker, with a good, strong, pleasing voice which is fully under your control and you must be able to hold what you are teaching in order to make its meaning clear. The fact that a good teacher has some of the gifts of a good actor doesn't mean that he will indeed be able to act well on the stage, for there are very important differences between the teacher's work and the actor's. The actor has to speak words which he has learnt by heart; he has to repeat exactly the same words each time he plays a certain part, even his movements and the ways in which he uses his voice are usually fixed beforehand. What he has to do is to make all these carefully learnt words and actions seem natural on the stage. A good teacher works in quite a different way. His audience takes an active part in his play: they ask and answer questions, they obey orders, and if they don't understand some thing, they say no. The teacher therefore has to suit his act to the needs of his audience, which is his class, he can not learn his part by heart, but must invent it as he goes along. What is the passage about?