I'm the first to admit I'm not a great conversationalist. That having been said, I think I'm at least somewhat proficient at gauging the appropriate response to what people say. Usually. I've never found the appropriate response to the following statement:
"You look tired."
Maybe my difficulty comes from not knowing why anyone would possibly want to say this to another person. Unless maybe they mean it as a not-very-witty insult? But that doesn't seem to be the typical motivation. In fact, judging by their tone, people seem to think this statement is somehow useful! As if maybe the tired-looking person wasn't aware? Here are some responses I've used:
"..." (that's me staring through (apparently) tired eyes, waiting for the rest of the sentence...because there must be more, right? Surely you're going to add something useful to this statement. Really, no? That's it, you're just sticking with "you look tired"?)
I think the sarcasm is appropriate, but I don't want to be witheringly sarcastic, just enough to make them think about what they've said. It usually doesn't work. If I come up with a REALLY good one, I'll stay up all night and see if I can solicit the comment, George Costanza style (the jerk store called...they're running out of YOU!). But I don't have my ultimate zinger yet...so I should go to bed.