The Flag of the United Mexican States

My girlfriend is in Mexico for the week. wrote my friend the Alchimst.

Throughout history, the flag has changed 4 times, as the design of the coat of arms and the length-width ratios of the flag have been modified. However, the coat of arms has had the same features throughout: an eagle, holding a serpent in its talon, is perched on top of a prickly pear cactus; the cactus is situated on a rock that rises above a lake. The coat of arms is derived from an Aztec legend that their gods told them to build a city where they spot an eagle and a serpent, which is now Mexico City. The current national flag, the Fourth National Flag, is also used as the Mexican naval ensign by ships registered in Mexico.