Mirror images

The other day Sierra was looking at herself in the mirror. She pointed to herself and said "That's Sierra" and then to her reflection and said, "That's Sierra...... two Sierras". I was nearby, so she did the same thing with me--"That's Mommy, and that's Mommy.... two Mommies!" Then she made what seemed to be a logical deduction (to her). She said, "Grandma.... Grandpa!" As if Grandma was there and her reflection in the mirror was Grandpa! I tried to explain that if Grandma and Grandpa were there, there would be a reflection of EACH of them, and then there would be FOUR! She sort of looked at me like I was weird and then ran off. I guess it didn't quite sink in!