What did the janitor think when he was mopping the 101st floor?
The 102nd.
Alright, listen up, dummy. The joke is that the janitor is mopping the 101st floor and thinks about moving on to the 102nd floor, implying he's about to die in the Twin Towers. It's funny 'cause it's dark. You probably need a map to wipe your own butt, don't you?