46   the saying goes, “Necessity is the mother ofinvention.” Many wonderful devices were developed just to  47 simpleneeds. The Post­-it note is a good example that can  48 thispoint.

    When Spencer Silver, the inventor of the Post­-itnote, was trying to invent a strong glue and experimenting with differentchemicals, he made a very weak  49   by accident. Then he began to think what aweak glue could be used for and was excited to tell others in his company   50    important his invention might be. They did notbelieve him,  51  .But later Art Fry, Silver’s co­-worker, proved that the weak glue was useful.One day, as Fry was   52 thathis bookmarks kept falling out of his hymn book, he thought, “If I could markthe right page without damaging the book, that  53   great.” Suddenly, he came up with the idea ofusing Silver’s weak glue  54 apiece of paper, and that worked! Fry shared this experience with his fellow (同儕的) workers and the company decided to   55   his idea into   55  .Thus, the Posit­-it note was born and then successfully spread around theworld.