There is a kingdom where if a person drinks poison he will die. The only way to counteract the poison is to drink a stronger one. Then the reaction stops. The king decides that he must have the strongest poison available in his possession. So, he sets up a contest between his court adviser and his wizard. Each must find the strongest poison in the kingdom and give it to him. But to be sure that he will get the strongest poison, he will force each to first drink the others' poison. Then they will drink their own. One of them will die, but the king will certainly then have the strongest poison in his possession. The court adviser knows that he can never outsmart the wizard, and bemoans the fact that in a few days time he will die. But soon, he and his wife think of a plan to outsmart the wizard. The day of the contest arrives. The wizard drinks the advicer's poison, and vice-versa. Then they drink their own and the wizard dies. What was the advicer's trick?