Anti-Lie Robot

One day, a father buys a robot that slaps people who lie.
He tests it at dinner.

DAD: Son, where were you today?
SON: At school.
*Robot slaps the son*

SON: Okay, I went to the movies. 
DAD: Which one? 
SON: Toy Story.
*Robot slaps the son again*

SON: Okay, it was Day with a Porn Star. 
DAD: When I was your age I didn't even know what porn was!
*Robot slaps Dad*

MOM: HAHA! After all he's your son.
*Robot slaps Mom*