Sunnytech Low Temperature Stirling Engine

Set this little engine on a mug of hot coffee, give the gold flywheel a gentle push, and it keeps spinning on nothing but the heat rising out of your cup. When it slows down, the coffee has cooled enough to drink, which makes it the most overbuilt drink timer on any desk.

It’s a low-temperature Stirling engine, a heat engine most people only meet as a diagram in a thermodynamics class. Sunnytech builds it with a mirror-finish stainless steel base, a glass cylinder liner, a precision bearing, steel shafts and a zinc alloy flywheel, and it runs without oil. It starts with a temperature difference of about 20 degrees Celsius and can reach 180 to 200 RPM. It works the other way around too: set it on an ice cube, or put it over hot water with a few ice cubes on the top plate, and the bigger difference makes it spin faster.

At roughly 10 by 10 by 14 centimeters and 250 grams, it fits beside a keyboard. It suits the mechanical engineer who still gets a kick out of thermo, a physics teacher, or anyone who likes a desk object that actually does something.

The catch: it only runs while there’s a temperature difference, so it winds down as your drink cools unless you keep a heat source under it. Keep it off stovetops and away from flames, because some of the parts are plastic and won’t survive direct heat.