Once upon a time there was a gentleman who led a very programmed life. He planned for every eventuality with utter care. He made sure to always have a plan B, just in case.
Every night before going to bed he placed on his night stand two glasses: one with water and one empty. When his visiting friend asked him “why?” he said:
“The one with water is there in case I wake up at night and I feel thirsty”.
Still not getting the point, the curious friend asked: “How about the other one?”
“Oh! This is in case I wake up at night and I am not thirsty!” the gentleman replied.