This is one of the questions my four-year old cousin asked my Grandma this morning, when he came over to play.
(He also asked her "Are you older than my mom?" which led to a visit to one of the family photo albums (The one with pictures of his mother as a baby.))
Little kids ask lots of questions, it seems, because they're still trying to figure out how the world works. Most adults have already given up.
(Or they've somehow gotten the dangerous notion that they understand it all, which can lead to some pretty magnificent missteps.)
So hurray for questions, and may we never stop asking them!