A medieval 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 gave him this command: "Prophet, tell me when you will die!"

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