A billion years ago when I was a saturday girl at a womens dress shop, a little girl came in with her dad and asked him, 'daddy why is that lady wearing that on her head?' he told her to ask me. I told her I because I was having a bad hair day, her dad laughed and she seemed to accept the reply.
I don't think you need to get heavy and meaningful about it.
I like Sr Jannahs reply, or you could say because its part of my religion...but that gets complicated.
And somedays, I do feel very glad I wear hijab, because when one does the school run at 7:30am one does not wish to spend several hours before hand sorting ones hair out!