First, it's a sing-song "uh-oh!" when something "falls" to the ground from J.J.'s highchair, stroller, car seat, etc.
It's followed by a sweet, emphatic, "Than-duh!" (that's "thank you" in J.J.-speak) when I hand him whatever it was he threw down just so I could pick it up for him. Well, I pick it up and hand it back to him the first time, just in case it was an accident. :)