Saw this joke on my cousin's KakaoStory profile. My rough line-by-line translation follows. I'm pretty sure I've heard this joke in English before, but it's still kinda' funny.
아빠가 거짓말 탐지기를 샀어요.
Dad bought a lie detector.
이 거짓말탐지기는 거짓말을 하는 사람을 때리는 기계예요.
This lie detector was a machine that would hit the person who lied.
아빠는 아들에게 이 기계를 테스트해보기로 했어요.
Dad decided to test the machine out on his son.
"너 어제 어디 있었니? " "도서관에 있었어요"
로봇이 아들을 때렸어요. "네, 친구집에 있었어요."
"Where were you yesterday?" "At the library."
The robot hit the son. "Yeah, I was at my friend's house."
"뭐했는데?"하고 아빠가 물어봤어요."토이스토리(애니메이션 영화)를 봤어요"
로봇이 아들을 때렸어요 "네, 포르노를 봤어요!" 아들이 소리쳤어요.
"What'd you do?" asked Dad. "I watched Toy Story (animated film)."
The robot hit the son. "Okay, I watched pornos!" the son shouted.
아빠가 화를 내며 말했어요 "뭐라고? 내가 너 나이에는 포르노를 알지도 못했어!" 로봇이 아빠를 때렸어요.
Dad got angry and said, "What? I didn't even know about pornos at your age!"*
And the robot hit Dad.
엄마가 웃으면서 말했어요. "역시 당신 아들이예요!"
Mom, laughing, said, "And he's your son, too!"
로봇이 엄마를 때렸어요
The robot hit Mom.
*For shits and giggles, I just ran this line through Google Translate. The result: "I did not even know you have older porn!" Never trust Google Translate.