(by Richard N. Zare but some confusing parts have been paraphased)
One day a fox was travelling through the forest, when she entered into a clearing and saw a rabbit busily working a typewriter. Naturally, this unusual phenomenon intrigued the fox. So she approached the rabbit and instead of immediately eating the her, asked what was going on.
"I'm typing my thesis," said the rabbit.
"What is the topic of your thesis," asked the fox.
"It's called Why Rabbits Eat Foxes."
"That's crazy," said the fox, "everyone knows that foxes eat rabbits, not the other way around."
"Why don't we step into my den and discuss this," the rabbit said, and the fox agreed. Time passed. Soon the rabbit emerged from the den, but the fox did not.
The following day, a wolf came into the same clearing. Same story, except this time the thesis title was Why Rabbits Eat Wolves. Same outcome: the rabbit came out of the den but the wolf did not. The day after, there was a similar occurance with a weasel.
All this time an owl had been watching all this, she wondered why all the other animals had not come out, and said to herself "I must see what is going on here." So the owl crept up very quietly to the entrance of the rabbit's hole and peered in. After her eyes became accustomed to the darkness, she saw in one corner a neat pile of fox bones. Nearby, another pile of wolf bones. And in the middle of the den was an enormous, mean-looking Lion, who was just finishing a nice weasel dinner.
Moral: It is not the thesis topic that matters; it is choosing the right advisor.