Lie detector robot

So a Dad builds a lie detecting robot that slaps anyone when they lie. He's been suspicious of what his son has been up to lately.

So they are sitting at the dinner table, the mum, the dad, the son, and the robot.
The dad asks his son "So where were you last night?"
The son replies "I was at the library"
*the robot slaps the son*
Son: "Fine, I was at Geoff's house"
Dad: "And what were you two doing?"
Son: "studying of course"
*robot slaps the son*
Son: "ok we were watching movies"
*robot slaps the son again*
Son: "Fine! we were watching porn ok?"
Dad: "I'm disappointed in you, when I was your age we didn't even know what porn was."
*the robot slaps the dad*
Embarrassed the Dad says "Well I certainly wasn't watching it!"
*the robot slaps the dad again*
The Mum starts chuckling at the end of the table and says "He's definitely your son"
*And the robot slaps the Mum"