An English professor was trying to make extra money selling vacuum cleaners door-to-door. At his next scheduled stop, he knocked and a little girl answered.
“Young lady, may I please have a word with your mother or father?” he asked.
“Nah..., they ain’t home, is they?” the girl replied. "They was in, but now they's out."
Taken aback, the man scolded the little girl: “Young lady, where’s your grammar?”
“She’s upstairs having a bath.”
The page I got it from also has another rather wonderful grammar joke (the second one), and a link to this brief page of "going into a bar" gramar jokes (I think number six is my favourit).