A Poison Duel

Once upon a time there was a land where the only antidote to a poison was a stronger poison, which needed to be the next drink after the first poison. In this land, a malevolent dragon challenges the country’s wise king to a duel. The king has no choice but to accept.

By bribing the judges, the dragon succeeds in establishing the following rules of the duel: Each dueler brings a full cup. First they must drink half of their opponent’s cup and then they must drink half of their own cup.

The dragon wanted these rules because he is able to fly to a volcano, where the strongest poison in the country is located. The king doesn’t have the dragon’s abilities, so there is no way he can get the strongest poison. The dragon is confident of winning because he will bring the stronger poison.

The only advantage the king has is that the dragon is dumb and straightforward. The king correctly predicts what the dragon will do. How can the king kill the dragon and survive?



  1. OMF:

    Well, by bringing his own up and filling it with just plain water, the King can kill the dragon.

    And by drinking a lesser poison beforehand, the Kind can also survive.

    – Before the duel, the King drinks the lesser poison and is poisoned. The duel then begins.
    – The King drinks the dragon’s poison, and is cured as this is stronger. The dragon drinks the King’s water, which has no effect.
    – Next, the King drinks his water, which has no effect. The dragon drinks his own poison, and is now poisoned.

    Everyone lives happily ever after, except the Dragon of course.

  2. Christ:

    He swaps the cups, and/or builds up a tolerance to iokane powder.

  3. adamas:

    “as you wish.”

  4. Cruel:

    The king gets the dragon to agree to drink first, then stokes the furnaces until hot enough to melt down the cups. The cups are heated till liquid. The dragon is forced to swallow molten metal and dies of the injuries.
    Be carful how you phrase things, the old celts were extraordinarily tricky about that.

  5. Gibbie:

    1. King takes lesser poison right before the duel.
    2. The duel begins, they trade cups, King drinks the full cup of stronger poison and is cured, the dragon drinks half cup of lesser poison.
    3. The dragon finds the King laughing because the strong poison cup is empty, they King drank it all, and now the dragon will die.
    4. The dragon eats the King who has the stronger poison inside him, and now the stronger poison is in the dragon and cures him.
    5. Dragon lives happily ever after.

  6. roza:

    I prefer first answer. please tell right answer.

  7. Adam:

    Since the correct answer was predictably immediately posted, let me suggest an alternative:

    1. King brings cup of regular poison, but in a custom made, extremely fancy cup.
    2. King boasts loudly that the poison is imported from the richer, more advanced neighbour of the kingdom and thus obviously is much better than the crap they sell you around here.
    3. Dragon being dumb and straightforward becomes scared and suggests to change the rules so that you drink from your own cup first.
    4. King magnanimously agrees

  8. gwern:

    > – The King drinks the dragon’s poison, and is cured as this is stronger. The dragon drinks the King’s water, which has no effect.

    How does this work? The King drinks the dragon’s poison-lava, and is cured of his pre-duel poison, sure; but then he is killed by the poison-lava.

    Nothing in the problem said that the escalation did anything to the current poison’s effects. Otherwise, this system is crazy and arbitrary: oxygen is a poison at not terribly high concentrations, so does that mean everyone in this kingdom can at every moment in time go ‘I just took some weak poison in the form of oxygen’ and then eat lava, random mushrooms, rotten food, etc.?!

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