Here's mine favorite one ...
Chuang Tzu once saw a dream that he has become a butterfly. And in the morning he was sad. And his disciples asked, "Why are you so sad?" Chuang Tzu said, "I am in trouble. In such a trouble I have never been before. This puzzle seems to be impossible; it cannot be solved. Last night I saw a dream that I have become a butterfly!"
The disciples laughed. They said, "What is there? This is not a riddle. A dream is just a dream." Chuang Tzu says, "But listen. I am troubled. If Chuang Tzu can dream that he has become a butterfly, a butterfly may be dreaming now that she has become Chuang Tzu. So how to decide whether I am now facing reality or again a dream? And if Chuang Tzu can become a butterfly, why can't a butterfly dream that she has become a Chuang Tzu?"