 Clausius’ version transparently rules out anti-thermodynamic behavior such as a hot iron bar extracting heat from a neighboring cold iron bar. The cool bar cannot give up a quantity of heat to the warmer bar (without something else happening). Kelvin’s statement is perhaps less obvious. It originates in an observation about steam engines, namely, that heat energy is a “poor” grade of energy. Consider a gas-filled cylinder with a frictionless piston holding the gas down at one end. If we put a flame under the cylinder, the gas will expand and the piston can perform work, e.g., it might move a ball. However, we can never convert the heat energy straight into work without some other effect occurring. In this case, the gas occupies a larger volume.