The Knight, The Dragon, and Seven Wells

A dragon has been terrorising the kingdom, so the king sends his finest knight to slay it. However, the dragon is too powerful to defeat by force, and the knight knows it. Instead, he seeks the dragon out and attempts to negotiate an alternative duel.

The dragon scoffs, but agrees to settle things with a challenge. On the island where the dragon makes its lair, there are seven poisoned wells, numbered 1 through 7. Each well’s poison is lethal within a day — unless the drinker takes water from a higher-numbered well before then, which neutralises the poison completely. Well 7, the highest, has no cure: anyone poisoned by its waters will die. The dragon’s challenge is simple. Tomorrow, they will each bring a chalice of liquid to the beach. They will exchange chalices and drink. Whoever survives wins; whoever dies loses.

The knight accepts the challenge. But when he scouts the island, he discovers a problem: the seventh well sits at the peak of a steep mountain that only the dragon can reach. The dragon had known this all along and set up a trap for him! However, the knight doesn’t give up.

The next day, the knight and the dragon meet at the beach, exchange chalices, and drink. The following day, the knight lives and the dragon dies. How?

Hint

From the dragon’s point of view, what would be his winning strategy? How can the knight counter that?

Solution

Before the duel, the knight drinks from one of the wells (example, well 1). He then brings a chalice of plain water.

The dragon would bring poison from well 7, knowing full well that the knight cannot cure the poison from that well. However, the water from well 7 instead cured the knight’s poison from the lower-numbered well.

The dragon would also suspect that the knight had brought poison from one of the 6 lower wells. He drinks from well 7 to attempt to cure himself, but ends up poisoning himself instead.