2011.12.22 When do babies say "no"?

Evan turned 15 months two days ago and is cuter than ever.  In addition to his vocabulary of probably 100 words, Evan has recently started to express himself with gestures. The most prominent gesture is his shaking of the head when expressing "no" or something in the negative.  For example, when looking at pages of a book, I can ask him if "moo moo" (cow) is there.  And he would shake his head emphatically and say "no".  For "yes", he doesn't yet nod his head. Instead, he'll confirm what you ask.  So with the cow example, if he see a cow in the pages, he'll say "moo moo, moo moo" and point to the cow.  Too cute!   If he is unsure of something or curious of what he's seeing, he'll tilt his head sideways and say "hmm?" or "huh?"  With physical and verbal gestures of yes/no, it's now a lot easier to communicate with him.  Like we'll now know if he really wants to eat something, or not..  He'll sometimes even say "bao", meaning he's full, and start taking off his bib to indicate that he's done eating and wants to get off the high chair.  What will he do next??