Is it a watermelon or is it an ellipsoid?
Ellipsoids, which are more or less a watermelon shape, are important in econometrics. In my Ph.D. Econometrics class at Harvard, Dale Jorgenson explained the effect of linear constraints by saying that slicing a plane through an ellipsoid would be like slicing a watermelon. Slices of a 3-dimensional ellipse–a watermelon–are in the shape of a 2-dimensional ellipse–a watermelon slice. Dale’s analogy of watermelons and watermelon slices inspired me to exclaim that slicing a 4-dimensional ellipsoid with a hyperplane would get you a whole watermelon!