Place the base piece on a smooth horizontal surface. The base alone balances readily. Next slip the cap piece over the pin mounted on the base with the Pb cavity on the long side of the base. The pair will be in a some what delicate balance (stable equilibrium).
Now rotate the cap 180° such that the Pb cavity is positioned on the short side. As a result of the rotation the center of gravity of the system is no longer directly over the area of support and the system will no longer balance on the angled end of the base.