The question Warren Buffett used to ask every CEO...

From Charlie Munger: The Complete Investor:

Munger is a believer in the investment approaches and ideas of Philip Fisher. Fisher was a successful investor based in California who wrote an influential book entitled Common Stocks and Uncommon Profits, first published in 1958. One of these ideas is that the successful investor is usually inherently interested in business problems. It is precisely for this reason that Ben Graham said that the best approach to investing is to be businesslike. The idea is that, in order to understand the stock, you must understand that the business is fundamental to the Graham value investing system. For this reason, investors like Fisher and Munger developed a “scuttlebutt” network of people who can help them to learn more about a business. What they inevitably find is that people involved in an industry will talk freely about their competitors as long as they believe they will not be quoted. Buffett uses the same approach: 

I would go out and talk to customers, suppliers, and maybe ex-employees in some cases. Everybody. Every time I was interested in an industry, say it was coal, I would go around and see every coal company. I would ask every CEO, “If you could only buy stock in one company that was not your own, which one would it be and why?” You piece those things together, you learn about the business after a while. —WARREN BUFFETT, UNIVERSITY OF FLORIDA, 1998