The teacher was wonderful at her explanation, saying that my DS really loves numbers and letters, so he works and plays with them a lot, just like how the little boy who asked the question really loved cars and trucks so he knew all about them.
That sounds like a perfect way to put it. I know that although my son is a very very strong reader, unless he loves a book, he won't necessarily blast through it and read at the detail level as when he does (devours somethings, rereads, talks about the book for several days).
My ds is 9, we have talked to him about possibly being gifted, and either way, just different, when he started noticing he wasn't fitting in, around 1st grade, and then immediately talked about why you don't go around running and telling everyone you're 'gifted'.
