A man buys a lie detector robot that slaps people who lie. he tested it at dinner:

Dad: Son were you at school today?
Son: Yes! (WHAP! robot slaps son)

Son: Okay I went to the movies!
Dad: Which one?
Son: Harry Potter (WHAP! robot slaps again!)

Son: Okay I was watching porn.
Dad: What? When I was your age I didn’t even know what porn was! (WHAP! robot slaps dad)
Mom: Hahahaha! after all he is your son!(WHAP WHAP!! robot slaps mom).