Kontax S90 stirling Engine

This is what I gave my father this Christmas, a fully functioning model of a stirling engine.
A stirling engine works on the pressure differences caused by temperature differences between bodies of gas/air. Read more about stirling engines here. This model is a small-scale beta stirling engine.

The set comes as a kit to build by yourself, but pre-built packages are available too. After construction, which is quick and easy, you can run the engine from the warmth of your hand (provided you have warm hands) or you can run it on top of a cup of hot water. Longest running time we got so far was well over one hour!

In case I got you interested is this kit; you can get it from Kontax in the UK.