Imagine you’re the head of a widget company, trying to put together a productive widget-making staff. It costs $20,000 to hire an entry-level widget maker, who produces 1,000 widgets a year. Since widgets sell for $15 apiece, you run a loss on the entry-level employees.

But, by the time your widget maker has had a few years of training and experience, she can produce 3,000 widgets a year. You make a huge profit.