Funny Dirty Joke

A man buys a lie detector robot that slaps people who lie. So he decides to try it out at dinner.
DAD: Son, where were you today during school?
SON: At school *robot slaps son*
SON: Ok, I went to the movies.
DAD: Which one?
SON: Toy Story *robot slaps son again*
SON: Ok, it was A Day with a Porn Star.
DAD: WHAT?! When I was your age, I didn't even know what porn was. *Robot slaps dad*
MOM: HAHA!! After all he is your son, *Robot slaps mom*