Okay, I got one for you all. Years ago, in school, I saw a propellor shaped thing, mirror side up, black side down. It was resting on a pin in, what I assume was, a vacuumed bell jar. It spins in sunlight as per what you'd expect... But what is the physics behind this? As light never slows due to energy loss, and light is massless, it can't be through that mechanism. The light never changes wavelength is no energy is lost that way. Maybe the force which drives it is proportional to the light absorbed by the mirror and blown out by the black surface? I'd like to know for sure thanks.