Good morning, class. Today we're going to discuss the history of the postal service. Many people assume that the first postal systems were developed in Europe, but in fact, the earliest recorded courier service dates back to ancient China around 2000 BC. However, the most famous early system was the Persian one, established by Cyrus the Great in the 6th century BC. This system used relay stations where riders could change horses, allowing messages to travel extremely quickly. Interestingly, the Persian system was so efficient that it was later adopted by the Romans, who expanded it across their empire. Now, let me ask you a question: what was the key feature of the Persian postal system?