Biological motion is for a large part powered by motor proteins. These are tiny engines (about a millionth of an inch) that convert chemical energy into mechanical force and motion. Processive motor proteins are among the most sophisticated and well studied of the motor proteins. They consist of two identical 'feet' that literally step forward on a long polymer as the fuel is consumed. When a human or other large animal steps, the physics involves mass, gravity and inertia. But for the walking protein the physics is different. Inertial forces are negligible compared to the frictional forces and Brownian motion, i.e. the random movements of molecules at a microscopic level, becomes an issue. Much of the research of the last decades has been directed towards figuring out the amino acid sequence and three-dimensional structure of proteins, but less effort and progress has been made towards understanding the operation of proteins in action. With a simple but rigorous model it is shown how Brownian motion and the generation of real force actually team up to make the motor protein step. The model, moreover, accounts accurately for recently obtained data on moving motor proteins.