BSP vs. depth-sort

Show that the back-to-front display order determined by traversing a BSP tree is not necessarily the same as the back-to-front order determined by depth-sort, even when no polygons are split. To receive full mark on this problem, the number of polygons you use for your example must be the smallest possible and you also need to prove that the number of polygons you used is the smallest possible. Hint: think about an example.