It took me awhile to figure it out, but when I was back in college, I took an economics course and our textbook was McConnell & Brue's Principles of Microeconomics. It was there that I found a favored passage that I refer to from time to time:
Human capital is the accumulation of outcomes of prior investments in education, training and other factors that increase productivity and earnings. It is the stock of knowledge, know-how, and skills that enables individuals to be productive and thus earn income. A valuable stock of human capital, together with a strong demand for one's services, can add up to a large capacity to earn income. For some people, high earnings have little to do with actual hours of work and much to do with their tremendous skill, which reflects their accumulated stock of human capital.
The point is demonstrated in the following story: It is said that a tourist once spotted the famous Spanish artist Pablo Picasso (1881-1973) in a Paris cafe. The tourist asked Picasso if he would do a sketch of his wife for pay. Picasso sketched the wife in a matter of minutes and said, "That will be 10,000 Francs." Hearing the high price, the tourist became irritated, saying "But that took you only a few minutes."
"No," replied Picasso, "it took me my entire life!"