自主学习：
Step 1. Introduction
Many animals are of great help to our daily lives or make great contributions to the development of science and technology. Can you list some of them?
Animals are friends of human beings. However, some animals will cause trouble or danger to us in some situations. For example, when they are hungry or when they feel themselves in danger. Do you know some dangerous animals on the land? There are also some dangerous animals in the water.
What do you know about sharks?
• Most of them have a weak sense of ______ but a strong sense of ______.
• Sharks must keep __________ so that they will sink.
Step 2. Fast Reading
• Do all the sharks attack human beings?
