My friend Paul will never forget his first chemistry teacher．He was a little man with thick glasses, but he had a strange way of making his classes lively and interesting．And his lessons were not easy to forget．

　　Paul remembers one of his first lessons from this teacher．After the students were all in the chemistry lab, the teacher brought out three bottles．One was petrol, one with castor oil(蓖麻油)and one with vinegar．“Now, watch carefully,”said the teacher．

　　He then filled a cup with some of the petrol, some of the castor oil and some of the vinegar．As the students watched him quietly, he mixed the three together．After that, he held up one of his fingers and showed it to the class．He then dipped it into the cup．After a few seconds he took his finger out．“Now watch,”he said．“Remember, you must do everything as I do．”

　　He put a finger in his mouth, tasted it and smiled, looking rather pleased．Then he handed the cup around the class of students．Each student dipped a finger into the mixture and sucked, then made a face．The mixture tasted terrible．

　　When the cup was at last returned to the teacher, he said sadly,“I’m sorry, none of you watched carefully enough．Yes, I sucked a finger, but the finger I put into my mouth was not the one I had dipped into the cup．”

　　It was Paul’s first important lesson as a student of chemistry and he never forgot it．

任务导入：Do you enjoy studying science?Explain why or why not．Some students are more interested in science than others．The reason for it is very simple．That is, they have an interesting chemistry class at the very beginning of their learning science．Have a discussion with your classmates about it．(和同班同学讨论你对科学感兴趣或不感兴趣的原因。)