Two excellent classic jokes are the "painted porch" joke and the "dog problem" joke. The painted porch joke tells the story of a man who has fallen on hard times and is wandering the streets of a wealthy neighborhood.
The man approaches one of the houses and knocks on the door and when the owner of the house answers, the homeless man asks him for a meal. The owner replies that he would be happy to feed the man in exchange for some honest labor. He tells the homeless man to go into the garage, get a can of paint from the shelf, and put a fresh coat of it on his porch. Fifteen minutes later, the wealthy man hears a knock at his door again. When he opens the door, he is greeted again by the homeless man who says, "Hey I'm all finished up with that paint job, but you should know that's a BMW in there, not a Porsche."
In the dog problem joke, a man walks into a psychiatrist's office in a state of great distress. The doctor asks him what the matter is and the man replies, "Well, I keep having these vivid hallucinations that I'm a dog! You've got to help me doctor, it's ruining my life!" "Settle down," replies the psychiatrist, "Just lie down on the couch over there, and maybe we can start to get to the bottom of this." "Oh no, I can't do that," says the man, "I'm not allowed on the furniture!"