A father buys a lie detector robot that slaps you when you lie. He decides to test it out on his son at supper.

"Where were you last night?"
"I was at the library."
*robot slaps son*

"OK I was at a friends house."
"Doing what?" asked the father. "Watching a movie. Toy Story."
*robot slaps son*

"OK it was porn!" cried the son. Father yells "What? When I was your age I didn't know what porn was."
*robot slaps the father*

The mother laughs and says "He certainly is your son."
*robot slaps the mother.

Great, but that's not a joke, that's life !