Why is this the answer?

The answer is musician because this clever riddle describes a person who has a very special kind of job. Think about what a musician does. They spend their time creating and performing music, whether they are playing an instrument like a guitar or a violin, or singing with a beautiful voice. For most people, when they play an instrument or sing a song, they are doing it for fun, which is why we use the word "play" when we talk about music. This is where the first part of the riddle, "plays when he works," makes sense. A musician's career is their "work" because they get paid for it and it is their profession. But because their work is making wonderful sounds and entertaining people, it feels like they are simply "playing" all day long. They are doing their job, but they are enjoying it just like you enjoy a game at recess. Now let's look at the second part: "works when he plays." To be a good musician, you cannot just play for fun all the time. A professional musician must practice for many hours a day. They have to learn complicated songs, repeat difficult musical parts over and over, and train their hands and voice. This kind of dedicated practice is not always easy; it takes discipline, effort, and concentration. Even though they are "playing" their instrument, that hard practice is actually difficult "work." It is the hard labor they must do to prepare for their performances. The riddle is a smart way to show that for a musician, their passion and their job are completely mixed up. Their "work" is fun like "play," and their "play" is serious like "work." They are one of the few people who can truly say that they love their job so much it feels like they are playing all the time.