A recently developed general theory of mechanical efficiency has yielded some new and interesting theoretical insights into the intrinsic performance limits of reciprocating heat engines. Various aspects of this theory have been presented in a series of papers at the last six IECEC meetings.This paper shows how the theory can be put to practical use in guiding the design of a particular engine. The subject engine is a Stirling intended to operate on a very small temperature difference across the warm and cool sides. The project afforded an extreme case to which the new theoretical results could be applied.The end product of the project is a small kinematic Stirling engine capable of operating down to a temperature differential of only 1/2 degree Celsius, or less than 1 degree Fahrenheit. The engine easily runs from heat absorbed while resting on the palm of a human hand. It begins running within about 15 seconds of being picked up, and after a few minutes reaches steady state operation at about 80 rpm. The engine is extremely entertaining and invaluable for teaching and demonstration.