Neither observation is consistent with a focus on the chemical properties of the "fuel". More qualitatively, the behavior of molecular motors made to work against an opposing force that exceeds their stall force, Fstall, can be inconsistent with our macroscopic expectations. For example, in the case of ATP-synthase, in which mechanical motion and ATP-hydrolysis/formation are strongly coupled, forcing it to move backwards reverses the direction of the coupled reaction; ADP and Pi are converted to ATP. Although a detailed discussion of the features ATPsynthase that lead to this behavior lies beyond this Perspective, it is not consistent with our experience of "fueled" motion in the macroscopic worlda car does not consume CO2 and produce gasoline if it is allowed to roll backwards downhill, the macroscopic equivalent of the behavior of ATP-synthase.