>You get the information you need from the context, as is the case with
>virtually all communication. How can I be sure that a 'thank you' is
>not an expression of sarcasm? From the context again.
I have to disagree on that - the context is sometimes not sufficient to understand the message; sometimes you simply need to think of a better, I mean more explicit, way of expressing a factual account of events.
Liz killed the man with a knife.
The man was killed with a knife by Liz.
The second sentence doesn't produce ambiguity, unlike the first one. And it was only rephrased to achieve the effect.