My niece is smack in the middle of that super-inquisitive phase. The two most common phrases out of her mouth are "what's that?" and "why?" She was in my room over yom tov, inquisitively looking through all my things.
3YON: Who's that?
SD: it's my friend.
3YON: what's she wearing?
SD: a gown.
3YON: why's she wearing a gown?
SD: cuz she's a kallah!
3YON: why's she a kallah?
SD: cuz she got married!
3YON: SD, why do your friends get married?
SD: I don't know cutie. I never got married. You better ask your mommy. SHE'S married!
(My sister-in-law still hasn't spoken to me since I made her explain THAT to a question-everything three year old but I'm hoping that by the time cutie starts high school...)