A dad buys a lie detector robot that slaps you if you lie.

Dad: Son, where were you at school hours? Son: At school. The robot slaps the son. Son: Okay I was watching KungFu Panda! The robot slaps his son again. Son: Okay I was watching violent movies! Dad: What?! When I was your age I never watched those kinds of movies! The robot slaps the dad. Mom: Haha, after all, he is your son. The robot slaps the mom