Adrian Bejan's field, a 59-year-old professor of mechanical engineering at Duke, is making no small claim when he says that he has discovered another. He calls it the Constructal Law and says it predicts how everything -- from lightning bolts and rivers to plants, people and money -- flows through time and space. It explains why birds and airplanes can fly, how cities grow and epidemics spread. It unravels the mysteries of snowflakes and Egypt's pyramids, the evolution of written languages.