Make Money While You Sleep

One day, a young boy went to his mentor's home to ask him how he could become rich. The mentor looked at him and asked, "How hard can you work?", the boy smiled and said, "I work harder than anyone of my age". Then, the rich mentor asked him, "How hard do you work when you're asleep?"
Confused, the young boy said, "Nobody work while sleeping, right?". The rich mentor told him, "Well, you have to figure out how to work hard when you're sleeping."