A father buys a lie detector robot that slaps people when they lie.
He decides to test it out at dinner one night.
The father asks his son what he did that afternoon.
The son says, "I did some homework."
The robot slaps the son.
The son says, "Ok, Ok, I was at a friend's house watching movies."
Dad asks, "What movie did you watch?"
Son says, "Toy Story."
The robot slaps the son.
Son says, "Ok, Ok, we were watching p*rn."
Dad says, "What? At your age I didn't even know what p*rn was."
The robot slaps the father.
Mom laughs and says, "Well, he certainly is your son."
The robot slaps the mother.

Robot for sale.