A medieval Jewish astrologer prophesied to a king that his favorite 
mistress would soon die. Sure enough, the woman died a short time later. 
The king was outraged at the astrologer, certain that his prophecy had 
brought about the woman's death. He summoned the astrologer and commanded 
him: "Prophecy, tell me when you will die!" 

The astrologer realized that the king was planning to kill him 
immediately, no matter what answer he gave. "I do not know when I will 
die," he answered finally. "I only know that whenever I die, the king will 
die three days later."