you'll note that the first thing the gorilla does when it sees the chest-thumping is that it interprets it as a communication signal and checks for danger. when it notices there's nothing there, it demonstrates an embarrassed reaction that more or less means "shit. you got me.". that is, it interprets the display as a type of practical joke and feels foolish for responding to it. what the gorilla is then doing is trying to figure out what the fuck the kid is doing - you can see the eyebrows furrow down in a process of contemplation. it's only when the kid points that the gorilla gets pissed, interpreting the little punk as a little punk and ready to smack him for being disrespectful. alas, the glass...