Apparently this little gismo has been used to introduce students to the Stirling engine, but I've never seen it. Stirling engines work with a sealed chamber and a piston where one side is hot and the other side cool. So if you place this engine on top of a really steaming hot cup of coffee, it will crank. And if you place this engine on top of a really cold plate of ice chips, it will crank... the opposite direction!
Stirling engines are extremely efficient, entirely clean, but so far they have been impractical to use because in order to be powerful they need a really wide variation in heat - more than just the difference between room temperature and the temperature inside a coffee cup. Apparently they also suffer from taking a long time to get up to speed.
Stirling engines were thought of 160 years ago as an alternative to then-dangerous steam power.
They don't have to look like hobbyist/kit stuff.
I really have to wonder if Douglas Adams (r.i.p.) knew about this coffee cup engine when he wrote (in the <i>Hitchhiker's Guide to the Galaxy</i>) about the infinite improbability drive requiring a really hot cup of tea to operate!