As a kid, Susan Athey tagged along when her grandfather sold cattle at auction. "I'd wonder, Why is this how they sell cows?" she recalls. At 38, she's now Microsoft's chief economist and a Harvard prof, asking grown-up versions of that question. Athey studies everything from government procurement to online ad sales. "How do you design an auction?" she asks. "How does design affect the health of the platform?" In 2007, she became the first woman to win the Clark Medal, a prestigious economics award. But her goal is less to win prizes than "to help make more efficient markets." That requires all kinds of research: She recently bought her son a toy train -- on eBay. -- by Jeff Chu