Determine what causes the PPF to take that shape. There is only one factor of production and it cannot be used in both sectors at the same time. Factors of production used to produce widgets cannot be used to produce gizmos, and vice versa. Factors of production can be shifted from widget production to gizmo production with no change in productivity. Factors of production can be used in either widget or gizmo production, but some are more productive at producing widgets and some are more productive at producing gizmos.