There are 2,027 bridges in New York City. On this week's edition of Please Explain, we'll learn how bridges work, and how they serve the communities that build them. Khaled Mahmoud, bridge engineer and president of Bridge Technology Consulting, joins us. He's also chairman of the Bridge Engineering Association. Also, Henry Petroski, Professor of Civil Engineering and History at Duke University. Mr. Petroski is also the author of several books, including Engineers of Dreams, a history of America's great bridges, and the forthcoming Success Through Failure: The Paradox of Design.