Some Things to Think About

So every time somebody asks me what I'm majoring in at University, and I tell them Philosophy, I get the same response: A puzzled look, which is usually followed by the question "What is that?" or "What will you ever possibly be able to do with that?" So, for those of you who don't know, philosophy deals with some of the following questions (but is most certainly not limited to these):

Do we see colours differently? Is my red your green? Is your blue my blue? How can we ever know? 

Even if I know nothing, I at least know that i know nothing. Thus meaning that I actually do know one thing. Which also means I'm back to zero, as the thing that I thought I knew is actually not true, causing me to know nothing at all.

Is a key still a key if there's nothing to unlock?

If you can not get something for nothing is it possible to get nothing for something?