If you want to know what's happening in the world, follow the heat. Why can't your coffee "steal" heat from the air to stay piping hot? Why can't Detroit make a car that's 100 percent efficient? Why can't some genius make a perpetual motion machine? The answers lie in the field of thermodynamics, the study of heat, which turns out to be the key to an astonishing number of scientific puzzles, including why time inexorably runs in only one direction...