One builds on petra, rock bed, rock bed. Not petros, that’s boulder or stone. Petra, rock bed. One has a concrete, if you will, or stone foundation; the other builds on sand. The Greek word for sand is ammon, for which the capital city of Jordan where I have visited a number of times gets its name: Amman, Jordan. It is aptly named. It is one big sand pile – Amman, Jordan.