The Sky

The sky is such a weird thing. I love it when you're in an airplane, flying through the clouds, but I still can't figure out one thing. What is the sky. Is it the clouds, or is it outer space? What is it actually? While I have never spent a lot of time pondering this, it suddenly came to me that it is possible the sky is not real. In reality, it is only a reflection of the ocean, so how does the sky work, and how did it come to be what we know now?