When the paper-cup salesman saw the lines at a Walgreen Drug's counter, he envisioned a new market. Why not sell milkshakes in paper cups and let customers carry them out? Store managers thought he sounded thicker than their shakes. They said that the extra penny and a half for the cup would eat up most of the profit on the 15? drinks. The young man finally won out by supplying free cups for a month. Sure enough, the take-out milkshake became a big winner, most of all for the salesman, Ray Kroc, who made plenty of paper-cup commissions. And his subsequent venture, McDonald's Corp., has sold millions of shakes -- all in paper cups, of course.