Father buys a lie detector that makes a loud beep whenever somebody lies around it. The son comes home in the afternoon. Father asks him, ""So, you were at school today, right?"" Son: ""Yeah."" Detector: ""Beep."" Son: ""OK, OK, I was in a cinema."" Detector: ""Beep."" Son: ""Alright, I went for a beer with my friends."" Father: ""What?! At your age, I wouldn't touch alcohol!"" Detector: ""Beep."" Mother laughs: ""Ha ha ha, well, he really is your son!"" Detector: ""Beep.""