The king of Sparta had another king visiting him. The king took his guest around and showed him his kingdom. He was particularly proud of Sparta's city walls, which, he claimed were impregnable.
The royal guest was curious and wanted to have a good look at those city walls. But nowhere in Sparta could he find them. As a matter of fact, Sparta had no city walls at all.
So he remarked to his host, "Excuse me, your majesty, but you spoke of impregnable city walls. Where are those city walls? I have looked all over Sparta and have found no walls at all."
At that the King of Sparta pointed to his people and said, "They are Sparta's walls. Each of them is a living stone in that wall."