一、于无疑处设疑,强化思维。《水的净化》一课,文中设计了一个简易过滤池,其过滤结构为上下两层:从底向上第一层为小石块,并从小石块的间隙处引出一个出水孔;第二层为沙子,进水孔在过滤池的顶端,需要净化的水从过滤池上端进水孔注入,依次经过沙子、石块的过滤后从底部的出口处流出。针对这一结构特点,老师可反问学生:能把出水孔和进水孔的位置 I. Set doubts at no doubt and strengthen thinking. In the “Water Purification” lesson, a simple filter tank is designed in the paper. Its filtering structure consists of two layers: a small stone from the bottom to the first layer, and a water outlet hole from the gap between the small stones; For sand, the inlet hole is at the top of the filter tank, and the water to be purified is injected from the inlet hole at the upper end of the filter tank. The sand and stones are filtered and then flow out from the outlet of the bottom. For this structural feature, the teacher can ask the students: The position of the outlet hole and the inlet hole