A snake says ssssssss

I'm sure that most of these posts are completely uninteresting and pointless for anyone other than myself who reads this, but bear with me.  This is serving as a great way for me to keep track of Isaiah's progress.  This past week marked some milestones with Isaiah's communication.  At the end of meals Isaiah if asked "all done?" will sign back to me the sign for all done.  When I ask Isaiah what a snake says he will say "sssssss."  If I ask Isaiah what a train says he pumps his arm up and down   and says "ch ch ch ch."  There is no vowel sound with it yet.  If I ask Isaiah what a Lion says he will make kind of a soundless growl or rather a hissing sound.  It's kind of a growl without sound.  Isaiah will also point to things and say "this."  The other day I was laying on the couch with Isaiah and I was scrolling through the photos on my blackberry and with each photo I was saying a little one line saying about it.  I think Isaiah thought I was reading to him.  Anyway he saw a picture of Mark, pointed to it, and said "da."  I think he knows dada goes with Mark but just chooses not to use it at he moment.  While these are all very exciting it seems that once he really has a new learned skill down he stops doing whatever he was doing.  Like now that the weekend is here I can't for the life of me get him to sign all done at the end of a meal.  Oh well we will keep encouraging!