The sin in Genesis 11:1-9 is that people sought to become like God. They wanted to be equal with Him and have the same knowledge and power. This was an act of rebellion against God and showed a lack of faith in His ability to govern the world. It's important to note that this story isn't about building a tower or achieving some great feat. It's about people trying to take control of their own lives and run things their own way, without reference to or respect for God. That's always been humanity's downfall, and it still is today. We think we know better than God what's best for us, and we try to go our own way.