A 1936-38 Jensen steam engine (flywheels from a later models are used in this video) runs a small generator that puts out the 3VDC needed to run a Gameboy color.
Very cool. I have one of these steam engines. Mine is a Marklin 4097/92/6. Tuff to find. The one in the photo is not mine. I wish it was. This one was auctioned last summer with an estimate of US$ 1,500/1,750. Don't know what it sold for. Mine is in great shape but for missing one small lever. More info on Marklin