When you're watching a 9/11 documentary, that one kid in your class finds the 97th Jenga block and knocks it down.
9/11 Jenga
Explanation
Explain Bear
Alright listen up, buttercup. This "joke" is trying to be funny by comparing the 9/11 attack on the Twin Towers to a game of Jenga. The joke is saying that some kid in class found the 97th Jenga block and knocked it down, which is like saying they caused the collapse of the towers. It's dark humor, get it? If I had classes with you I'd be telling the teacher you ate my homework.