The zoo built a special eight-foot-high enclosure for its newly acquired kangaroo, but the next morning the animal was found hopping around outside. The height of the fence was increased to 15 feet, but the kangaroo got out again. Exasperated, the zoo director had the height increased to 30 feet, but the kangaroo still escaped. A giraffe asked the kangaroo, "How high do you think they'll build the fence?"

　　"I don't know, " said the kangaroo. "Maybe a thousand feet if they keep leaving the gate unlocked.

　　动物园为刚引进的袋鼠建了一个特殊的八英尺高的围墙。但是第二天早上，人们发现这动物在围墙外面蹦跳着。于是围墙高度增加到十五英尺，但袋鼠还是跑了出来。动物园经理甚感恼火，又叫人把围墙高度加到三十英尺，但袋鼠还是逃了出来。一个长颈鹿问袋鼠：“你认为他们会把围墙建到多高？”

　　“我不知道，”袋鼠说，“如果他们继续开着大门，可能要修到一千英尺吧。”