The ellipse makes up all objects that have a flat circular plane, such as the top rim of a vase, a round plate surface, the bottom of a glass - well, you get the picture. Understanding how a circle behaves when viewed from different horizons is key to accurately rendering these objects.
Experiment looking at a cylinder, such as a round vase or glass, from directly above. Then move away so that you are looking across the top of the cylinder but still above it. Then examine the cylinder at the "eye level" (in line with the top of the cylinder) as shown in the diagram above. Finally, situate yourself so that your eye level is below the top of the cylinder. By doing this simple exercise you will be able to identify the ellipses that occur in your still lifes.
Take note that when you view a cylinder from directly overhead, you will see an undistorted circle. As your eye level drops across the top of the cylinder, the circle will transform into an ellipse that becomes shorter and flatter as your eye level approaches the level at the top of the object. When your eye level is directly in line with the top of the cylinder, the ellipse seen previously disappears and becomes a straight horizontal line.
This lesson will help you "see" that the top and bottom of a tall cylinder will appear to be two ellipses of the (nearly) same width but of different heights.