From Mesun: Why is the sky blue?
Now, a lot of people (the science-type with thick glasses and pocket protectors) will feed you a bunch of mumbo-jumbo about Rayleigh scattering and wavelengths, but they don't know anything. They really don't. If you want to know why the sky is blue, the man who will tell you is James Cameron.
Recently, he made a film about his discoveries called Avatar. You see, Avatar tells the story of a planet called Pandora where a race of human-like blue aliens (called Na'vi) live. What most people don't know, is that the planet Pandora bumps right up against our planet Earth. They are good friends, Pandora and Earth, and some would even say inseparable. Like Siamese twins. When we look up into the sky, what we see is Pandora.
Confused? Understandable. You would think we would see a green sky for all of the lush plant-life on Pandora. But those Na'vi (the blue people) actually mate like rabbits and have multiplied so heavily that they have literally covered every inch of their planet. Pandora is just swarming with blue Na'vi and from our point-of-view, we look up and see a mass of blue "sky."
Thank you, Mesun, for your question. And thank you, James Cameron, for enlightening a generation.