This discussion is informative and interesting, but wasn't the original question about "okay, Hello, Yes/Maybe"? As in, do these represent whole sentences and not a thought? I don't really get how this topic turned into a competition to find the shortest sentence in the world.
Whoever said the shortest was nothing is wrong, because mathematically the smallest number from 1-100 isn't zero. It's 1. You can't assume that the absence of something is less than that given item's worth. This is actually different in situations where zero or less is considered a part of the whole series. For example, I have 0 dollars. I have less money than someone who has $400. You see? It all depends.
As for this topic, I'm still confused about whether "Hello" is a thought or sentence. Can we focus more on this?