Fine summer evening, I'm taking Sempi on a walk. He's carrying a flash light and shining it on every piece of object on the side walk and asking me questions.
Sempi: Appa, what is this?
Me: It's a rock.
Sempi: Oh. What is this?
Me: That's a piece of paper.
Sempi: Oh. What is this?
Me: It's a cigarette. Actually it's a cigarette butt.
Sempi: Huh? Do you keep it in the butt?
Me: Hahahaha. No. Sempi. *Still laughing uncontrollably*
Sempi: Hehehe. *Starts laughing*
He knows when he's made a joke, even though he has no clue why it's funny.