Paddy and Murphy are walking down the street when all of a sudden, Paddy falls down a manhole. Murphy shouts down, "Paddy, is it dark down there?"
Paddy shouts up, "Dunno Murphy, I crnt see a fecking thing!"
Alright, listen up, genius. So, Paddy falls into a manhole, right? Murphy asks if it's dark down there, and Paddy's all like, "I can't see a fecking thing!" The joke is that if Paddy can't see anything, he wouldn't know if it's dark or not, would he? It's funny because it's so stupid. Get it? Probably not, you walnut.