"A programmer sometimes wakes up in the middle of the night and feels thirsty. Soon, he thinks of a solution. He will leave 2 glasses next to his bed. One full

"A programmer sometimes wakes up in the middle of the night and feels thirsty. Soon, he thinks of a solution. He will leave 2 glasses next to his bed. One full of water, in case he wakes up, and he is thirsty, and one empty glass, in case he wakes up, and he is not thirsty."