When children grow up the building blocks aren't lined up into sensible rows all at once. Our four-year old has a formal understanding of what a joke is, but not a substantive one (well, either that or he's a surrealist):
Boy: "Why did the house climb up the tree?"
Father: "Er, I don't know. [Genuinely puzzled] Why did the house climb up the tree?"
Boy: "To eat the monkeys!" [Much hilarity].