The lie detector

Father buys a lie detector that makes a loud beep whenever somebody tells a lie. 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 at the movies.” 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! He really is your son!” Detector: “Beep.”