To start this conversation I ask my kiddos to tell me what they know about 9/11. We listen and talk. As the conversation turns into more questions than facts I play a video from BrainPOP.
The video is normally the featured video on Sept. 11th, so even if you don't have a BrainPop subscription you can still show the video for free.
How do you explain 9/11 to your students? What are your favorite activities to do with your kiddos to help them deal with such a heavy topic?