The students were having their chemistry (化学) class. Miss Li was telling the children what water was like. After that, she asked her students,"What ’ s water?" No one spoke for a few minutes . Miss Li asked again "Why don't you answer my question ? Didn't I tell you what water is like?" Just then a boy put up his hand and said, "Miss Li , you told us that water has no colour and no smell . But where to find such kind of water? The water in the river behind my house is always black and it has a bad smell." Most of the children agreed with him . " I'm sorry , children." said the teacher, "Our water is getting dirtier and dirtier . That's a problem .