What a five year old notices? I can tell you at least one. Here's a conversation I just had with mine.
him: Mom, I gotta tell you something, but it's kind of weird.
me: What? (he should have been in bed an hour ago).
him: I have TWO shadows! One in the front and one in the back.
I couldn't help it, I laughed, and then tried to explain why. I think he feels better about it now, but he was a little disappointed at first.