Another oldie but goodie. Approximately 6" tall evacuated clear glass bulb mounted in a black stand, 3" in diameter at the top. A four bladed vane is balanced on a needle inside the sphere. Because opposite surfaces of the vanes are black and white, incident radiant energy is absorbed or reflected to a different degree. The result is a spinning vane. The brighter the light, the faster it spins. Simple, but compelling. Brief explanatory instruction sheet included.