A man buys a new type of lie-detecting robot that slaps people when they tell a lie. He decides to test it at dinner. He asks his son, "Son, where were you today during school hours?"
"Oh, I was at school, as usual, Dad," replies the son, whereupon the robot strides over and slaps the son on the cheek.
"Okay, okay, I went to Johnny's house and we watched a movie," says the son.
"Which movie?" asks the father.
"Harry Potter, Dad," whereupon the robot slaps the son again, only harder this time.
"Ow! " exclaims the son, "Okay, okay, we were watching porn."
"WHAT?! " roars the father. "When I was your age I didn’t even know what porn was! " whereupon the robot strides over and slaps the father.
Now the mother starts laughing: "Hahahahahaha! Why are you surprised? After all, he is your son! " whereupon the robot strides over and gives her a slap.
The next day there is an ad on Craigslist: "For sale: one lie-detecting robot. Cheap."