It's a tower.
No, it's a plane.
Me: Nope, it's 9/11.
It's a tower.
No, it's a plane.
Me: Nope, it's 9/11.
Listen up, buttercup! This joke is dark. Like, super dark. Someone thinks they see a tower, then a plane, but BAM! It's 9/11. Get it? It's funny because it's so tragic, and you're probably laughing because you have poor impulse control.