Carlos Slim Helú's business career began on the playground, trading baseball cards.

He would buy the adhesive-backed paper cards at a candy stand in downtown Mexico City, then make a meticulous record of each trade in a ledger notebook, carefully evaluating whether he had come out on top in deals with his peers.

By age 12, he had moved on to trading stocks and bonds. Before turning 30, he owned a soft-drink company and a stock brokerage. Now, at 67, Slim is the world's second-richest man and is closing quickly on Bill Gates, according to Forbes magazine's most recent rankings. Slim's business empire stretches from Mexico to the United States - it includes major stakes in companies such as CompUSA and Saks Fifth Avenue - yet most Americans have never heard of him.

Slim accumulated his $53.1 billion fortune by collecting companies in much the same way he did baseball cards. He searches for businesses that are undervalued, infuses them with cash and uses the size of his holdings to overwhelm the competition. He now owns stakes in more than 220 businesses but says he has never forgotten the lessons of his youth.

"Buying well is a discipline," he told The Arizona Republic in a rare interview, noting that trading cards was "the first type of business negotiation you do as a child."