have you ever heard of a crookes radiometer? it’s a little gimmick, a glass campanula with a sort of windmill inside… except, it has no batteries at all and it’s not the wind making it go, it’s the sun!

there’s not much to it: four very lightweight panels resting on a pin, inside a glass bubble with gas at a specific pressure. one side of the panels is coated in black. as it heats up with the sun, it creates some temperature differential in the gas which makes the whole thing spin. the more the sun shines on it, the faster it goes! (here’s a more scientific explanation)

pretty neat, right? we have one sitting in a table that gets sunlight throughout the day, and don’t seem to get tired of seeing it spin. it definitely feels like magic! :)