ok. heres one.
there is one knight and one dragon trapped on an island.
the island has seven levels, 1 being the lowest.
all the water on that island is poison. the only way to cure the poison is to drink from a level higher eg. if you drink from level 1, every level above will cure you. (that level that cures you doesnt poison you as well.)
therefore drinking from level 7 will kill you (obviously)
the dragon can get to all the levels, and the knight can only get to level 6.
the dragon gives the knight a drink from level 7. the knight drinks it and does not die. he then gives the dragon a drink. the dragon drinks it, but he ies.
-what did the knight give the dragon to drink
-why didnt the knight die.
Presumably the Knight survived because he drank from level 6 or lower before getting the drink from the dragon (unless you poison yourself when you cure yourself, in which case ALL the water will kill you, effectively, but you said it doesn;t so that is ok.). Did he then somehow offer the dragon's own drink back to him?