Stolen from another forum:

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 schoolwork." 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 adult movies."

Dad says, "What? At your age I didn't even know what adult movies were." The
robot slaps the father.

Mom laughs and says, "Well, he certainly is your son." The robot
slaps the mother.

Ad appears on Ebay the next day: "Lie Detecting Robot for sale. Cheap."