I once got into a relatively lively email debate with a friend of mine concerning the nature of free will. One of us argued that we had it and the other argued that we didn't. The exchange ended when he sent me an email explaining that he was agitated and losing sleep over the matter.
Evelyn's birthday passed a few months ago. We were at a bar with her friends and I somehow got into a discussion with some guy about the nature of seeing green.
I'm sure the thought has run though many people's heads in some form or another. It basically goes like this: what if your green is not my green? Ignoring colour-blindness, when I look at the grass I register it as "green" and so do you. We are both, of course, able to verbalize this, but there's no way to tell if we're really seeing the same "green". What if "green" to you is really what I consider to be "red"? There'd be no way of knowing - you'd still call it "green", it's still the colour we both associate with grass, etc.