Lie detector robot

Dad buys a LIE DETECTOR ROBOT which slaps people when they lie. He decides to test it at diner.
"Son, where were you today?"
The son says "at school, dad." Robot the son!
"Ok, I watched a DVD at my friends house!"
"What DVD?"
"Toy story." Robot slaps the son again!
"Ok, it was a p**no" cries the son.
Dad yells "What! When I was your age I didn't know what p**no was!" Robot then slaps the dad!
Mom laughs "HaHaHa! He's certainly YOUR son." Robot then slaps the mom.