Often referred to as a 'solar windmill', this fascinating scientific curiosity was invented by the English chemist Sir William Crookes in 1873. When exposed to 'radiant light' such as sunlight or an incandescent light bulb the paddles inside will spin; the hotter the light, the faster they go! This all happens due to pressure imbalances created within the perfect vacuum of the glass bulb. Even if the science is a bit beyond you, it is a truly beguiling marvel to sit on a sunny windowsill & observe.