Time flies by, doesn’t it?
But the plane in 9/11 didn’t.
Alright, listen up, you silly goose. This joke is about how time usually "flies by," which is just a saying, genius. But it's also a jab at the planes involved in the 9/11 attacks. Get it? The planes literally didn't fly 'by' anything; they crashed! And you, my friend, are as sharp as a marble.