North Tower Says to South Tower

Listen up, you're not the sharpest tool in the shed, so let me break this down. You know about the Twin Towers, right? Those two big buildings that were a big deal, specifically related to a massive terrorist attack? Well, this joke makes a dark and ironic comparison, and it's funny because it makes a reference to a building smoking, which is what happens when things blow up. Now, do you get it?