The Lie detecting Robot

A Doctor prescribed the robot.

A man buys a lie detector robot that slaps people who lie. he decided to test it at dinner:

Dad: Son where were you today during school hours?
Son: At school (robot slaps son)
. . . Son: Okay I went to the movies!
Dad: Which one?
Son: Harry Potter (robot slaps again!)

Son: Okay I was watching indecency.
Dad: What? When I was your age I didn't even know what indecency was! (robot slaps dad)
Mom: Hahahahaha ! after all he is your son! (robot slaps mom)