There is a rule used almost subconsciously by almost all English speakers (and I'm sure it applies to many other languages too) which is that yes/no questions are asked ending with a rising tone, and others are ended with a falling tone. Why is this and how did the phenomenon develop?
There is one exception to this rule that I've found - if you ask a yes/no question with a falling tone, it is still understood as a question, but that you're asking it in a very unenthusiastic or grudging way. Again, is there any obvious reason for this or did it just kind of randomly develop?