Once upon a time there was a kingdom. A king and a clown lived in this kingdom. Unfortunately they hated each other so they agreed that they will poison each other one day.
There are only twelve vials of poison in whole kingdom and they are locked in one chamber in the castle.
The poisons have numbers from 1 to 12. The higher the number the stronger the poison. The effect on the human body is simple – you drink the poison, you die. Each stronger poison neutralizes all weaker poisons which means that poison 12 neutralizes all poisons, poison 11 neutralizes all poisons except poison 12, etc. If you drink poison 11 followed by poison 12 nothing happens. If you drink poison 12 and then poison 11 you die.
The king enters the chamber first and takes all the even numbered poisons (2, 4, 6, 8, 10, 12). The clown then enters and takes the odd numbered poisons. They meet in the throne hall where each fills one cup and hands it over to the other who immediately drinks it. Now each fills the cup once again, now for himself, and drinks it (hoping to save his own life).
Both the king and the clown primarily want to survive but want to poison the other. There is one dose of each poison – it’s not possible to divide it. The poisons are fluids without color or smell and they have the same consistency as water.
The clown survived and the king died from the poison. What did the clown do?
The king keeps poison 12 so that he can neutralize any poison that the clown gives him. He pours poison 10 (his second strongest poison) and gives it to the clown to drink.
Clown can deduce this so he keeps poison 11 for himself to neutralize poison 10. That’s how he survives.
He also knows that the king, after drinking the cup he gave him, will drink poison 12 to neutralize the poison. So the clown pours plain water instead of poison into the cup for the king. The king drinks the water (water is not poison) and right after that he drinks the strongest poison possible (12) and dies.