Knife, who?

Hey, you! Yeah, you, the one who probably still uses dial-up. This joke is funny because it starts like a normal "knock-knock" joke, but then it takes a dark turn when the knocker admits to stabbing the person on the other side of the door. The surprise and absurdity of the victim responding after being stabbed is what makes it funny... or at least darkly amusing. You probably think dark jokes are when your toast is burnt. Sad.