How did she get to be so smart?

Today we went to the beach at Kent park, pictures to follow later. When we got there Aubrey counted the people who were already there and announced that there were 5. She asked how many people there would be after we went down there. I asked her, "How many of us are there?" She answered, "Four." So I asked, "How many will there be?" She thought for a few minutes and just when I was about to give her a clue, she said, "Nine!" How did she get to be so smart?