Lie Detector

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.”