And that, connected with the existing river systems, allowed for a lot of trade to happen, especially as we get into the Tang and the Song Dynasties. Now in the Song Dynasty in particular, you have the pound lock being invented, which allows for easier travel across waterways when you have a change in the level of water. And the pound lock is still the standard technology used in canals throughout the world. You also have the first use of paper money. This is a big deal. We use paper money today. Our ability to have transactions in our society are not dependent on our ability to mine for gold or silver or copper.