Answer
Suppose a highlight should appear in the middle of a patch. That highlight is caused by a specific surface orientation at that point of the patch. Because Gouraud shading considers only the surface orientations along the boundaries of the patch, it will miss the highlight. But, because Phong shading attempts to determine the surface orientations within the patch interior, it may detect the highlight.
Work Step by Step
Suppose a highlight should appear in the middle of a patch. That highlight is caused by a specific surface orientation at that point of the patch. Because Gouraud shading considers only the surface orientations along the boundaries of the patch, it will miss the highlight. But, because Phong shading attempts to determine the surface orientations within the patch interior, it may detect the highlight.