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 clear speaker, with a good, strong, pleasing voice which is fully under your control; and you must be able to act what you are teaching, in order to make its meaning clear. Watch a good teacher, and you will see that he does not sit still before his class: he stands the whole time he is teaching; he walks about, using his arms, hands and fingers to help him in his explanations, and his face to express feelings. Listen to him, and you will hear the loudness, the quality (音色) and the musical note of his voice always changing according to what he is talking about. 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 (预先). 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 something, they say so. The teacher therefore has to meet the needs of his audience, which is his class. He cannot learn his part by heart, but must invent it as he goes along. I have known many teachers who were fine actors in class but were unable to take part in a stage-play because they could not keep strictly to what another had written.