There is one thing. We can't see it. Sometimes we can hear it and even feel( 感知 ) it. Without it, all the living things will die. Can you guess what it is? Yes, it is air 。 However, what's the air like today? In cities, there are too many people and too many cars on the roads. The cars send out( 排放 ) a lot of bad gases. In cities, there are many chemical (化学) factories. They send out terrible smoke every day. This kind of smoke is dangerous for our health. Now the air is becoming worse and worse. Every day, we breathe( 呼吸 )the bad air in and out. This makes us feel sick 。 So in order to keep us healthy, we have to go to the countryside to breathe more fresh air. Or we can go up hills to get more fresh air.