Roses are red,
Violets are blue,
I know a pedophile,
And he says he knows you.
Alright listen up, genius. This joke is funny because it rhymes. Bet you didn't know that "roses" and "knows" rhyme. It's also dark because it suggests that a pedophile is familiar with the listener, implying a disturbing connection or potential threat. You probably have a lot of friends, but I doubt they are all people you want to be associated with!