摘要：
Air is all around us. It's around us as we walk and play. From the time we were born, air was around us on every side. When we sit down, it is around us. We live in air. All living things need air. L... 阅读全文
摘要：
Water is very important for living things. Without water there can be no life on earth. All animals and plants need water. Man also needs water. Water is found almost everywhere. Even in the driest pa... 阅读全文