I recently had the chance to have coffee with my sister and my 10 month old niece.  As she was sitting in her highchair I was feeding her bits of an oatmeal cookie.  Each time I gave her a bite of the cookie I would sign the word cookie to her.  I was signing eat and cookie each time she had a piece.  She was watching me carefully to see what I was doing.  Within a matter of minutes, I noticed her little fingers moving in mock sign language.  Of course, she would not be signing back to me that quickly, but she could understand that the movement as I spoke the words was always the same and it was a special code I was using with her.  She has never been exposed to sign language, but she seemed to enjoy it.