A teacher asks his pupils, “Can anyone tell me the difference between constructive criticism and unnecessary nastiness?”
Little Johnny puts his hand up.
“One is the process of offering valid and well-reasoned opinions about the work of others, usually involving both positive and negative comments. “
“Very good, Johnny, “says the teacher. “And the other?”
“You are a cunt. “
This joke is included in the Black Book of Jokes. You can order it from here.