How can phrases mean things their words don't appear to mean?
For example, if I'm eating a salad, my friend asks how it is, and I say "not bad," the words "not bad" seem to be extremely open - the salad could be amazing, it could be okay, it could be great or it could be totally neutral; it might even be horrible, so long as it isn't "bad." However, I would normally be understood as saying the salad was okay, rather than any of the other logically plausible alternatives.
How does that work?