A dragon and a knight live on an island where the only sources of fresh water are a pond (containing ordinary water) and six wells, numbered 1 to 6. Each well's water is completely indistinguishable from the pond water, but contains a magical poison that has no immediate symptoms, yet suddenly kills the drinker about an hour after drinking.
To make things complicated still, each well contains a different variety of poison. If a drinker who has been poisoned by a well drinks the water from another well, the result depends on the relative numbers. If the second well has a higher number then both poisons will eliminate each other and the drinker will be cured. If the second well has the same number or a lower one, it is as though the drinker had only drunk from the first well. For example, if you drink from well 1 and then from well 4 you will not be poisoned, but if you drink from well 4 and then well 1 you will be poisoned as though you had drunk only from well 4.
As a result of these rules, water from well 6 can cure poison from any of the other wells, but, when drunk by someone who is not poisoned, is incurably lethal. Furthermore, while wells 1-5 are a short walking distance from each other, well 6 is located at the top of an unclimbable mountain on the island that the knight cannot reach but the dragon can fly onto very quickly.
This sets the stage for the following puzzle: both knight and dragon understand these rules completely, know the numbering of each of the wells, and each want the other dead. Being evenly matched in combat, they arrange a special sort of duel. Each secretly fills a glass of water from one of the island's sources, then meets the other in a field, where they exchange glasses, and drink. They may seek water from any of the island's sources that they can personally reach before and after.
Is it possible for either the knight or the dragon to ensure it will survive this duel?
Note: drinking the same poison twice in a row (or 20 times in a row) has exactly the same effect as drinking the poison once.
Answer
The knight can ensure survival
by drinking from well 4 first, and 3 + 5 after. If the dragon uses pond water, or 1, 2, 3 or 4, the last drink from 5 will neutralize the others (4, ? and 3). If the dragon gives 5 or 6, it will neutralize the first '4', then 3 will be neutralized by 5.
As AeJey said, the dragon can ensure survival by
Drinking from well 5 before, and 6 after the duel.
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