Lou Zhongjian, an associate professor of Chemistry of the Teacher's College in Sichuan Province, invented an excercise-paper that can be used repeatedly. He dips a writing brush in clear water then writes on a piece of grayish paper with it. What he writes on the paper immediately appears in black as if written with Chinese ink. But it disappears gradually as it dries. Then the paper can be used again. It is made of materials of plentiful supply and the cost is low.