Take the capital of a pre-Columbian empire - a city on an island in the middle of a lake - then, after it's destroyed by Spanish conquistadores, build another city on top of what's left. Then make the new city bigger and bigger as the lake dries up.
That is the recipe for both Mexico City and chaos.
On a recent trip to the city - now a sprawling metropolis of 21 million people -- I took an informal survey of what the local folks think are their top five must-see attractions. Here's what they said you shouldn't miss: