One day the staff members of a zoo called a meeting to discuss the problem–how to deal with the kangaroos that were found out of the cage. They came to the conclusion that the cage was placed too low and decided to raise it from one to two meters high. But the next day the kangaroos were still at large and they again raised the cage to three meters. Quite beyond their expectation the next morning they saw the kangaroos still free to go about. They were alarmed and determined to go to all the length by raising the cage to the height of ten meters. Later a giraffe, while chatting with some kangaroos, asked them, “Do you think they will go on raising your cage?” “Hard to say,” said a kangaroo, “if they continue forgetting to fasten the cage door.” 有一天怎么说，动物园的管理员们发现袋鼠从笼子里跑出来了，于是开会讨论，一致认为是笼子的高度过低，所以他们决定将笼子的高度由原来的1米加高到2米。结果第二天他们发现，袋鼠还是跑到外面来，所以他们又决定再将高度加高到3米。 没想到隔天居然又看到袋鼠全跑到外面，于是管理员们大为紧张，决定一不做二不休，将笼子的高度加高到10米。 长颈鹿和几只袋鼠在闲聊，“你们看，这些人会不会再继续加高你们的笼子?”长颈鹿问。 “很难说。”袋鼠说，“如果他们再继续忘记关门的话!”
