When I have used spheres and discs, I have intended that they should represent more than what they just are. More or less as the earth is a sphere, but also has some miles of gas about it, volcanoes upon it, and the moon making circles around it, and as the sun is a sphere—but also is a source of intense heat, the effect of which is felt at great distances. A ball of wood or a disc of metal is rather a dull object without this sense of something emanating from it.
(A. Calder, quoted in Museum of Modern Art Bulletin, vol. 18, no. 3, Spring 1951).