Kid A: What's the ninth letter of the alphabet?
Kid B (pause while fingers are counted on): I
Kid A: What colour is the sky?
Kid B: Blue
Kid A: What's the opposite of on?
Kid B: Off
Kid A: Euuuurrrrr! You blew off!
You blew off! Hey everyone, [Kid A] just said "I blew off"! This works best if you run around doing it to as many people as you can, because it's only a matter of time before everyone has heard it. And if that doesn't happen, there's only a brief period in your life when you will be childish enough for it to be funny.

Alternatively:
A: What's your name?
B: B
A: (Point to your nose) What's this?
B: Nose
A: (Hold your hand out) What am I carrying?
B: Nothing
A: B knows nothing! He said so himself! He's shouting it from the rooftops!