What is more useful after it is broken?

猜谜语：What is more useful after it is broken? What is more useful after it is broken? What is more useful after it is broken?Why is a river rich?W What is moreuseful after it is broken? What ------ useful advice it is! Learn what is more useful motto? It is ( ) useful book. It is useful book What is never used until it is broken? How a useful book it is!What tall building it is!What great How a useful book it is What tall building it is what great What is useful怎么回答