Categorical imperative
A hypothetical imperative might be that "a thirsty man must drink water if he wants to stop being thirsty."  If Thirsty Man lived by a maxim based on this hypothetical imperative, it might be "If I can, I will drink water whenever I am thirsty."   In this example, Thirsty Man is not making any obvious moral choice.  Some philosophers would say that the Thirsty Man's maxim is a reasonable one.  Based on Thirsty Man's maxim, he will soon drink the water.   A few minutes later a second man is brought into the room.  Both men are told that they will be in the room all night, and that no one else will be back to see them until morning.