Phong shading allows curved surfaces comprised of polygona to appear smooth. Unfortunately, phong shading also can produce some unexpected results. In the sample above, the same basic shape takes on different appearances when the layout of the facets is changed.
In order to emphasize the shading variations, I set the objects specularity to maximum, and its hardness to 50%.
As you can see, the apparent roundness can be noticabley varied by the placement of additional edges along the stretches from the top of the cup to bottom. In the first set of wireframe and shaded cup pairs, the barrel of the cup is composed of four sections. (I know, its sort of hard to see) The two middle sections are placed close to the to and bottom of the cylinder. The illusion of a curved areas is pushed towards the ends in this way. In the second paint, the middle segments have been placed closer to the middle, and one of the ring section in the glass' foot is moved closer to the stem.
The last two pairs have the limiting bands removed. In the third sample from the left, Phong shading is turned on. As you can see, the attempt is made to smooth the barrel of the glass, producing odd shading. The final examples have phong shading turned off at the edges of the barrel. This was done by using the Imagine command, Make Sharp Edges. The result, while not appearing very cup like, does clearly show the cylindrical nature of the section.
Therefore, when creating objects, place your poly edges carefully in order to avoid either "pillows" or "crinkles".