the lie detecting robot

A father buys a lie detector robot; it slaps people when they 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, OK," the boy says, rubbing his arm. "I was at Jerry's house."

"Doing what?" asks the mother.

"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 even know what porn
was!"

robot slaps the father

The mother laughs and says, "He certainly is your son!"

robot slaps the mother