A medieval wise man prophesied to a king that his favorite son would soon die.  Sure enough, the young man died a short time later.  The king was outraged at the wise man, certain that his prophecy had brought about the boy's death.  He summoned the wise man and commanded him: "Tell me when you will die!"

The wise man 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...."