6. How to defeat Koschei? In the sacred and dense, terrible Murom forest, magical springs of water gush from the ground. Water can be taken from the first nine springs by anyone, but the tenth spring is located in Koschei the Deathless's cave, and no one except Koschei himself can take water from there. The magical water tastes and looks no different from ordinary water, but if a person drinks from any of the springs, they will die (the magical water is even deadly to Koschei himself). The only thing that can save a person is to drink water from a magical spring with a higher number. However, if one drinks water from the tenth spring, nothing can help. Ivan the Prince challenged Koschei to a duel. The conditions are as follows: each brings a cup of water and gives it to their opponent to drink. Koschei agreed. He reasoned: "If I give Ivan the Prince water from the tenth spring, he will perish. If I then wash down his poison with the same water, I will be saved." How can Ivan the Prince win the duel?
Problem 516
Official solution
6. Ivan-the-Prince must drink magic water (from any source) before the duel, then Koshchei, giving him water from the tenth source, will save him from certain death. Koshchei, however, he must give plain water (not magical!) to drink. Koshchei, after drinking plain water and washing it down with water from the tenth source, will perish.