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.

　　動物園為剛引進的袋鼠建了一個特殊的八英尺高的圍墻。但是第二天早上，人們發(fā)現(xiàn)這動物在圍墻外面蹦跳著。于是圍墻高度增加到十五英尺，但袋鼠還是跑了出來。動物園經(jīng)理甚感惱火，又叫人把圍墻高度加到三十英尺，但袋鼠還是逃了出來。一個長頸鹿問袋鼠：“你認為他們會把圍墻建到多高？”

　　“我不知道，”袋鼠說，“如果他們繼續(xù)開著大門，可能要修到一千英尺吧。”