Assassin

Mr. Smith and his son Rick were called to Mrs. Liventhal's classroom.
"Mr. Smith," said the teacher, "I asked Rick 'Who shot Abraham Lincoln?' and he said that he didn't do it!"
"Well, teacher," said Smith, "if my kid said he didn't do it -- he didn't do it!"
Father and son left the school, and on their way home, Smith turned to the boy and asked, "Tell me, son, did you do it?"