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 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.”