In February 2015, shares of famed motorcycle manufacturer Harley-Davidson, Inc., were trading for about $62. At that price, Harley-Davidson had a price-earnings, or PE, ratio of 16, meaning that investors were willing to pay $16 for every dollar in income earned by Harley-Davidson. At the same time, investors were willing to pay $252 for each dollar earned by 3D printer company 3D Systems Corporation, but only a meager $6 and $9 for each dollar earned by Marathon Oil and Citigroup, respectively. And then there were stocks like Amazon which, despite having no earnings (a loss actually), had a stock price of about $370 per share. Meanwhile, the average stock in the Standard & Poor's (S&P) 500 index, which contains 500 of the largest publicly traded companies in the United States, had a PE ratio of about 17, so Harley-Davidson was average in this regard.

As we look at these numbers, an obvious question arises: Why do you think investors were willing to pay so much for a dollar of 3D Systems' earnings but so much less for a dollar earned by Harley-Davidson?