Yesterday, after we'd spent hours walking around Tate Modern:
Es Both my legs really hurt.
Matt Well, all mine do.
Es You can't say 'all' there.
Matt Why not? 'All' can still mean 'two'. Why isn't it my intonation that's wrong?
Es Because Grice's Maxims of Conversation say you shouldn't be ambiguous.
Mind you, the Maxims also say you shouldn't lie and you should always be relevant -- but they're a good starting point if you don't need to worry about sarcasm, humour, irony, and so on.