Actually Merci. Just this week I've been hearing Max say, "Merci" when I give him things. I wasn't quite sure what he was saying as toddlerese can be hard to decipher at times. But yesterday, I gave him something and I said to him "thank you" and he responded with "Merci".
Though it sounds more like Ah-chi. And sometime he says it in English and it comes out as "Ah-choo". Very cute all the same. Sure does make this sick day much better hearing him say it to me all the time. Like after I wipe his nose for the millionth time.