- If the bubble contains English text, that is letters that are not meant onomatopoetically then it must be a thought bubble. Animals cannot talk.
- However, animals
can "say" the sounds they ordinarily make. So, Snoopy can say "Arf" or "Ruff" or even "Bark."
- As we see here, Snoopy can also say
punctuation. Here I believe the intent is just to show a mood. Snoopy isn't actually saying anything, it is just showing his mental state. The word balloon is technically extraneous here.
- Once in a while you'll see Snoopy say something that is borderline between the two. Some time ago he said "Heh heh," which is hard to adjudicate. Dogs can't laugh, but presumably they can make sounds like being amused, so I assume that was Schulz's intent.
- Then there was that strip in which Snoopy had a sheet over his head and said "Boo." The joke in that one was from being inexplicable.