Perhaps it is his impending birthday (the big five), but Son has begun considering the practical aspects of existence lately.
Son: Mom, do I have to have a job?
Me: Well, most adults have jobs because they have to pay for things, like rent and food.
Son: Why don't some people have jobs?
Me: Well, sometimes people can't find jobs, or they quit their jobs, or they aren't working for other reasons.
Son: What's another possibility?
(later, when I was tucking him in bed)
Son: Mom, when I'm older, I might have to quit my job.
Son: I might, I might.
Son: When I'm a pilot, I'm going to have a mustache, because it makes you look like a real pilot, like on the cartoons.