Teotihuacan is pretty convenient to Mexico City. A short bus-ride from the Bus terminal Norte, for 36 Pesos, and the ticket seller gave me my first lesson in caveat emptor: "The bus leaves in three minutes," he informed us, "Do you have 8 pesos?" I had given him a 200-peso note, and fished out the requisite change. He gave us our three tickets and we hurried away.... You guessed it. He did not readily proffer our 100-peso-note change. The bus was pulling out of its parking space by the time we did the arithmetic, and realized we had been allowed to overpay.