What was special about the cities of the ancient Indus Valley?

About 4,500 years ago, along a river in what is now Pakistan and northwest India, people built cities so neat and clever that they still make modern planners a little jealous. We call them the Indus Valley cities, and the two most famous are Harappa and Mohenjo-daro. Here's the surprising part: these weren't messy clusters of huts. They were planned, top to bottom, before a single brick was laid.
Most old cities grow like weeds — a house here, a lane there, all crooked. But the Indus builders did the opposite. They drew straight streets that crossed each other at neat right angles, like a giant tic-tac-toe board. Walk one of their main roads and you could see clear to the other end. Someone, clearly, loved a good grid.
