Sigh, this is a oldie!

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.

The robot slaps the son.

Okay, I was at a friends house, the son admits.

Doing what? asked the father.

Watching a movie: Toy Story.

The robot slaps the son.

Okay, it was porn! cried the son.

Father yells What? When I was your age I didn’t know what porn was!

The robot slaps the father.

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

The robot slaps the mother.