It's a good thing, too. If everyone was like that ALL the time, we might all lose what's left of our minds.
Three-year-olds can jump from topic to topic with a mental dexterity that is both amazing and confusing. To wit:
Me: "You have a VERY dirty face. After dinner, guess where you're going?"
Little Brother: "Where?"
Me: "The BATH!!!"
Little Brother: "No! No! I'll take a bath SUNDAY!"
Me: "But you're dirty today. You need a bath today."
Little Brother: "But I ate ALL my grapes!"
Obviously if I am going to be forced to have conversations like this one, I'd better have a whole lot more coffee than I'm currently consuming.