@ balancing nails on a nail

Pound a large nail far enough into a board so that the nail stands securely upright. Lay a second large nail on a flat surface and place other nails across this nail, head to head as shown above. Finally, place another nail on top of this assembly, head to tail with the second nail. Carefully pick up the assembly and balance it on the upright nail.

In a gravitational field, any object is most stable when its center of mass is as low as possible. The center of mass of the nail assembly is below the point of support and at its lowest when the assembly is balanced. If the assembly swings to the side, its center of mass rises. Gravity that exerts a restoring force to bring the assembly back into balance.